diff --git a/benchmarks/ConstructionAndIteration/benchmark_composition_construction_and_iteration.cpp b/benchmarks/ConstructionAndIteration/benchmark_composition_construction_and_iteration.cpp
new file mode 100644
index 0000000..7cc559b
--- /dev/null
+++ b/benchmarks/ConstructionAndIteration/benchmark_composition_construction_and_iteration.cpp
@@ -0,0 +1,123 @@
+#include "benchmark_utils.h"
+
+#include "fourdst/composition/composition.h"
+#include "fourdst/atomic/species.h"
+
+#include <chrono>
+#include <random>
+#include <ranges>
+
+std::chrono::duration<double, std::nano> benchmark_construction(const size_t iterations, const size_t nSpecies) {
+    using namespace fourdst::composition;
+    using namespace fourdst::atomic;
+
+    // Setup random machine to get random double between 0 and 1 for molar abundances
+    std::random_device rd;
+    std::mt19937 gen(rd());
+    std::uniform_real_distribution<> dis(0.0, 1.0);
+
+    std::vector<Species> species_to_register;
+    std::vector<double> molarAbundances;
+
+    size_t count = 0;
+    for (const auto& sp : species | std::views::values) {
+        if (count >= nSpecies) {
+            break;
+        }
+        species_to_register.push_back(sp);
+        molarAbundances.push_back(dis(gen));
+        count++;
+    }
+
+    const auto duration = fdst_benchmark_function([&]() {
+        for (size_t i = 0; i < iterations; ++i) {
+            fourdst::composition::Composition comp(species_to_register, molarAbundances);
+        }
+    });
+
+    return duration / static_cast<double>(iterations);
+}
+
+std::chrono::duration<double, std::nano> benchmark_access(const size_t iterations, const size_t nSpecies) {
+    using namespace fourdst::composition;
+    using namespace fourdst::atomic;
+
+    // Setup random machine to get random double between 0 and 1 for molar abundances
+    std::random_device rd;
+    std::mt19937 gen(rd());
+    std::uniform_real_distribution<> dis(0.0, 1.0);
+
+    std::vector<Species> species_to_register;
+    std::vector<double> molarAbundances;
+
+    size_t count = 0;
+    for (const auto& sp : species | std::views::values) {
+        if (count >= nSpecies) {
+            break;
+        }
+        species_to_register.push_back(sp);
+        molarAbundances.push_back(dis(gen));
+        count++;
+    }
+
+    const Composition comp(species_to_register, molarAbundances);
+
+    std::uniform_int_distribution<>(0, nSpecies - 1);
+    std::vector<Species> random_lookup_species;
+    for (size_t i = 0; i < iterations; ++i) {
+        random_lookup_species.push_back(species_to_register[static_cast<size_t>(dis(gen))]);
+    }
+
+    const auto duration = fdst_benchmark_function([&]() {
+        for (size_t i = 0; i < iterations; ++i) {
+            volatile double y = comp.getMolarAbundance(random_lookup_species[i]);
+            do_not_optimize(y);
+        }
+    });
+
+
+    return duration / static_cast<double>(iterations);
+}
+
+int main () {
+    constexpr size_t nIterations = 1000;
+    constexpr size_t nSpecies = 100;
+
+    std::vector<double> durations;
+    durations.resize(nIterations);
+
+    for (size_t i = 0; i < nIterations; ++i) {
+        std::print("Iteration {}/{}\r", i + 1, nIterations);
+        auto duration = benchmark_construction(10, nSpecies);
+        durations[i] = duration.count();
+    }
+    std::println("");
+
+    std::println("Average time to construct composition over {} iterations: {} ns", nIterations,
+                 std::accumulate(durations.begin(), durations.end(), 0.0) / nIterations);
+    std::println("Max time to construct composition over {} iterations: {} ns", nIterations,
+                 *std::ranges::max_element(durations));
+    std::println("Min time to construct composition over {} iterations: {} ns", nIterations,
+                 *std::ranges::min_element(durations));
+
+
+    plot_ascii_histogram(durations, "Composition Construction Time Histogram");
+
+
+    durations.clear();
+    durations.resize(nIterations);
+    for (size_t i = 0; i < nIterations; ++i) {
+        std::print("Iteration {}/{}\r", i + 1, nIterations);
+        auto duration = benchmark_access(1000, nSpecies);
+        durations[i] = duration.count();
+    }
+    std::println("");
+    std::println("Average time to access composition over {} iterations: {} ns", nIterations,
+                 std::accumulate(durations.begin(), durations.end(), 0.0) / nIterations);
+    std::println("Max time to access composition over {} iterations: {} ns", nIterations,
+                 *std::ranges::max_element(durations));
+    std::println("Min time to access composition over {} iterations: {} ns", nIterations,
+                 *std::ranges::min_element(durations));
+
+    plot_ascii_histogram(durations, "Composition Access Time Histogram");
+}
\ No newline at end of file
diff --git a/benchmarks/ConstructionAndIteration/meson.build b/benchmarks/ConstructionAndIteration/meson.build
new file mode 100644
index 0000000..538d791
--- /dev/null
+++ b/benchmarks/ConstructionAndIteration/meson.build
@@ -0,0 +1 @@
+executable('construction_and_iteration_bench', 'benchmark_composition_construction_and_iteration.cpp', dependencies: [composition_dep], include_directories: [benchmark_utils_includes])
\ No newline at end of file
diff --git a/benchmarks/hashing/benchmark_composition_hash.cpp b/benchmarks/hashing/benchmark_composition_hash.cpp
index 0eeb108..a7e0990 100644
--- a/benchmarks/hashing/benchmark_composition_hash.cpp
+++ b/benchmarks/hashing/benchmark_composition_hash.cpp
@@ -1,58 +1,16 @@
 #include "fourdst/composition/composition.h"
 #include "fourdst/composition/utils/composition_hash.h"
-#include "fourdst/composition/utils.h"
 #include "fourdst/atomic/atomicSpecies.h"
 #include "fourdst/atomic/species.h"
 
-#include <chrono>
 #include <numeric>
 #include <print>
-#include <string>
 #include <vector>
-#include <cstdint>
 #include <ranges>
+#include <chrono>
 
-template <class T>
-void do_not_optimize(T&& datum) {
-    asm volatile("" : "+r" (datum));
-}
+#include "benchmark_utils.h"
 
-uint32_t calc_num_bins(const std::vector<double>& data) {
-    // Use Sturges' formula
-    const size_t n = data.size();
-    return static_cast<uint32_t>(std::ceil(std::log2(n) + 1));
-}
-
-std::string plot_ascii_histogram(std::vector<double> data, std::string title) {
-    // Use std::format
-    const uint32_t nBins = calc_num_bins(data);
-    const double minVal = *std::ranges::min_element(data);
-    const double maxVal = *std::ranges::max_element(data);
-
-    std::string histogram;
-    histogram += std::format("{:^60}\n", title);
-    histogram += std::string(60, '=') + "\n";
-    std::vector<uint32_t> bins(nBins, 0);
-    const double binWidth = (maxVal - minVal) / nBins;
-    for (const auto& value : data) {
-        const uint32_t binIndex = static_cast<uint32_t>((value - minVal) / binWidth);
-        if (binIndex < nBins) {
-            bins[binIndex]++;
-        } else {
-            bins[nBins - 1]++;
-        }
-    }
-    const uint32_t maxBinCount = *std::ranges::max_element(bins);
-    for (uint32_t i = 0; i < nBins; ++i) {
-        const double binStart = minVal + i * binWidth;
-        const double binEnd = binStart + binWidth;
-        const uint32_t barLength = static_cast<uint32_t>(std::round((static_cast<double>(bins[i]) / maxBinCount) * 50.0));
-        histogram += std::format("[{:.2e}, {:.2e}): {:>15} | {:}\n",
-                                 binStart, binEnd, bins[i], std::string(barLength, '*'));
-    }
-    return histogram;
-
-}
 
 std::chrono::duration<double, std::nano> build_and_hash_compositions(const size_t iter, const size_t nSpecies = 8) {
     using namespace fourdst::composition;
@@ -69,15 +27,14 @@ std::chrono::duration<double, std::nano> build_and_hash_compositions(const size_
         count++;
     }
 
-    const auto start = std::chrono::high_resolution_clock::now();
-    for (size_t i = 0; i < iter; ++i) {
-        uint64_t hashValue = utils::CompositionHash::hash_exact(comp);
-        do_not_optimize(hashValue);
-    }
-    const auto end = std::chrono::high_resolution_clock::now();
+    const auto duration = fdst_benchmark_function([&]() {
+        for (size_t i = 0; i < iter; ++i) {
+            uint64_t hashValue = utils::CompositionHash::hash_exact(comp);
+            do_not_optimize(hashValue);
+        }
+    });
 
-    const std::chrono::duration<double, std::nano> duration = (end - start)/iter;
-    return duration;
+    return duration / static_cast<double>(iter);
 }
 
 int main() {
diff --git a/benchmarks/hashing/meson.build b/benchmarks/hashing/meson.build
index d92b6d5..195a088 100644
--- a/benchmarks/hashing/meson.build
+++ b/benchmarks/hashing/meson.build
@@ -1 +1 @@
-executable('hashing_bench', 'benchmark_composition_hash.cpp', dependencies: [composition_dep])
\ No newline at end of file
+executable('hashing_bench', 'benchmark_composition_hash.cpp', dependencies: [composition_dep], include_directories: [benchmark_utils_includes])
\ No newline at end of file
diff --git a/benchmarks/meson.build b/benchmarks/meson.build
index 4b32e51..ad7f744 100644
--- a/benchmarks/meson.build
+++ b/benchmarks/meson.build
@@ -1 +1,4 @@
-subdir('hashing')
\ No newline at end of file
+benchmark_utils_includes = include_directories('utils')
+
+subdir('hashing')
+subdir('ConstructionAndIteration')
\ No newline at end of file
diff --git a/benchmarks/utils/benchmark_utils.h b/benchmarks/utils/benchmark_utils.h
new file mode 100644
index 0000000..ba72b16
--- /dev/null
+++ b/benchmarks/utils/benchmark_utils.h
@@ -0,0 +1,65 @@
+#pragma once
+
+#include <algorithm>
+#include <cstdint>
+#include <vector>
+#include <string>
+#include <cmath>
+#include <format>
+#include <chrono>
+
+
+template <class T>
+void do_not_optimize(T&& datum) {
+    asm volatile("" : "+r" (datum));
+}
+
+inline uint32_t calc_num_bins(const std::vector<double>& data) {
+    const size_t n = data.size();
+    return static_cast<uint32_t>(std::ceil(std::log2(n) + 1));
+}
+
+inline std::string plot_ascii_histogram(std::vector<double> data, std::string title) {
+    const uint32_t nBins = calc_num_bins(data);
+    const double minVal = *std::ranges::min_element(data);
+    const double maxVal = *std::ranges::max_element(data);
+
+    std::string histogram;
+    histogram += std::format("{:^60}\n", title);
+    histogram += std::string(60, '=') + "\n";
+    std::vector<uint32_t> bins(nBins, 0);
+    const double binWidth = (maxVal - minVal) / nBins;
+    for (const auto& value : data) {
+        const uint32_t binIndex = static_cast<uint32_t>((value - minVal) / binWidth);
+        if (binIndex < nBins) {
+            bins[binIndex]++;
+        } else {
+            bins[nBins - 1]++;
+        }
+    }
+    const uint32_t maxBinCount = *std::ranges::max_element(bins);
+    for (uint32_t i = 0; i < nBins; ++i) {
+        const double binStart = minVal + i * binWidth;
+        const double binEnd = binStart + binWidth;
+        const uint32_t barLength = static_cast<uint32_t>(std::round((static_cast<double>(bins[i]) / maxBinCount) * 50.0));
+        histogram += std::format("[{:.2e}, {:.2e}): {:>15} | {:}\n",
+                                 binStart, binEnd, bins[i], std::string(barLength, '*'));
+    }
+    return histogram;
+
+}
+
+template <typename Func>
+auto fdst_benchmark_function(Func&& func_call) {
+    auto start = std::chrono::high_resolution_clock::now();
+
+    // Forward the callable
+    std::forward<Func>(func_call)();
+
+    auto end = std::chrono::high_resolution_clock::now();
+    auto duration = std::chrono::duration_cast<std::chrono::nanoseconds>(end - start);
+
+    do_not_optimize(duration.count());
+
+    return duration;
+}
\ No newline at end of file
diff --git a/src/composition/include/fourdst/atomic/atomicSpecies.h b/src/composition/include/fourdst/atomic/atomicSpecies.h
index b374d44..058db27 100644
--- a/src/composition/include/fourdst/atomic/atomicSpecies.h
+++ b/src/composition/include/fourdst/atomic/atomicSpecies.h
@@ -1,6 +1,7 @@
 #pragma once
 
 
+#include <format>
 #include <string_view>
 #include <string>
 #include <optional>
@@ -275,8 +276,7 @@ namespace fourdst::atomic {
 
         friend bool operator==(const Species& lhs, const Species& rhs);
         friend bool operator!=(const Species& lhs, const Species& rhs);
-        friend bool operator<(const Species& lhs, const Species& rhs);
-        friend bool operator>(const Species& lhs, const Species& rhs);
+        friend std::partial_ordering operator<=>(const Species &lhs, const Species &rhs);
     };
     /**
      * @brief Equality operator for Species. Compares based on name.
@@ -296,25 +296,18 @@ namespace fourdst::atomic {
     inline bool operator!=(const Species& lhs, const Species& rhs) {
         return (lhs.m_name != rhs.m_name);
     }
-    /**
-     * @brief Less-than operator for Species. Compares based on atomic mass.
-     * @param lhs The left-hand side Species.
-     * @param rhs The right-hand side Species.
-     * @return `true` if lhs atomic mass is less than rhs atomic mass, `false` otherwise.
-     */
-    inline bool operator<(const Species& lhs, const Species& rhs) {
-        return (lhs.m_atomicMass < rhs.m_atomicMass);
-    }
-    /**
-     * @brief Greater-than operator for Species. Compares based on atomic mass.
-     * @param lhs The left-hand side Species.
-     * @param rhs The right-hand side Species.
-     * @return `true` if lhs atomic mass is greater than rhs atomic mass, `false` otherwise.
-     */
-    inline bool operator>(const Species& lhs, const Species& rhs) {
-        return (lhs.m_atomicMass > rhs.m_atomicMass);
+
+    inline std::partial_ordering operator<=>(const Species &lhs, const Species &rhs) {
+        if (const auto cmp = lhs.m_atomicMass <=> rhs.m_atomicMass; cmp != 0) {
+            return cmp;
+        }
+
+        return lhs.m_name <=> rhs.m_name;
     }
 
+
+
+
     /**
      * @brief Converts a spin-parity string (JPI string) to a double-precision floating-point number representing the spin.
      *
@@ -424,14 +417,15 @@ namespace fourdst::atomic {
  * }
  * @endcode
  */
+
 template<>
 struct std::hash<fourdst::atomic::Species> {
     /**
-     * @brief Computes the hash for a Species object.
-     * @param s The Species object to hash.
-     * @return The hash value of the species' name.
-     */
+         * @brief Computes the hash for a Species object.
+         * @param s The Species object to hash.
+         * @return The hash value of the species' name.
+         */
     size_t operator()(const fourdst::atomic::Species& s) const noexcept {
         return std::hash<std::string>()(s.m_name);
     }
-}; // namespace std
+};
diff --git a/src/composition/include/fourdst/composition/composition.h b/src/composition/include/fourdst/composition/composition.h
index 1968c2f..2a26aa8 100644
--- a/src/composition/include/fourdst/composition/composition.h
+++ b/src/composition/include/fourdst/composition/composition.h
@@ -26,6 +26,7 @@
 
 #include <optional>
 #include <unordered_set>
+#include <expected>
 
 #include "fourdst/config/config.h"
 #include "fourdst/logging/logging.h"
@@ -96,6 +97,9 @@ namespace fourdst::composition {
      */
     // ReSharper disable once CppClassCanBeFinal
     class Composition final : public CompositionAbstract {
+    public:
+        using iterator = detail::CompositionIterator<false>;
+        using const_iterator = detail::CompositionIterator<true>;
     private:
         /**
          * @struct CompositionCache
@@ -138,6 +142,11 @@ namespace fourdst::composition {
                        !Ye.has_value() && !sortedSpecies.has_value() && !hash.has_value();
             }
         };
+
+        enum class SpeciesIndexLookupError : uint8_t {
+            NO_REGISTERED_SPECIES,
+            SPECIES_NOT_FOUND
+        };
     private:
         /**
          * @brief Gets the logger instance for the Composition class. This is static to ensure that all composition
@@ -149,11 +158,18 @@ namespace fourdst::composition {
             return logger;
         }
 
-        std::set<atomic::Species> m_registeredSpecies; ///< Set of registered species in the composition.
-        std::map<atomic::Species, double> m_molarAbundances; ///< Map of species to their molar abundances.
+        // std::set<atomic::Species> m_registeredSpecies; ///< Set of registered species in the composition.
+        // std::map<atomic::Species, double> m_molarAbundances; ///< Map of species to their molar abundances.
+
+        std::vector<atomic::Species> m_species;
+        std::vector<double> m_molarAbundances;
 
         mutable CompositionCache m_cache; ///< Cache for computed properties to avoid redundant calculations.
 
+    private:
+        [[nodiscard]] std::expected<std::ptrdiff_t, SpeciesIndexLookupError> findSpeciesIndex(const atomic::Species &species) const noexcept;
+        [[nodiscard]] static std::vector<atomic::Species> symbolVectorToSpeciesVector(const std::vector<std::string>& symbols);
+
     public:
         /**
          * @brief Default constructor.
@@ -295,6 +311,7 @@ namespace fourdst::composition {
          * @return A reference to this Composition.
          */
         Composition& operator=(Composition const& other);
+        Composition& operator=(const CompositionAbstract& other);
 
         /**
          * @brief Registers a new symbol for inclusion in the composition.
@@ -546,7 +563,7 @@ namespace fourdst::composition {
          * value of this method will only be valid as long as the Composition object is valid (i.e. it cannot
          * outlive the Composition object it was called on).
          */
-        [[nodiscard]] const std::set<atomic::Species> &getRegisteredSpecies() const noexcept override;
+        [[nodiscard]] const std::vector<atomic::Species> &getRegisteredSpecies() const noexcept override;
 
         /**
          * @brief Gets the mass fractions of all species in the composition.
@@ -767,8 +784,8 @@ namespace fourdst::composition {
          * for species are defined based on their atomic mass. When iterating over the molar abundance map, species will be
          * seen in order from lightest to heaviest.
          */
-        [[nodiscard]] std::map<atomic::Species, double>::iterator begin() override {
-            return m_molarAbundances.begin();
+        [[nodiscard]] iterator begin() override {
+            return {m_species.begin(), m_molarAbundances.begin()};
         }
 
         /**
@@ -788,8 +805,8 @@ namespace fourdst::composition {
          * for species are defined based on their atomic mass. When iterating over the molar abundance map, species will be
          * seen in order from lightest to heaviest.
          */
-        [[nodiscard]] std::map<atomic::Species, double>::const_iterator begin() const override {
-            return m_molarAbundances.cbegin();
+        [[nodiscard]] const_iterator begin() const override {
+            return {m_species.cbegin(), m_molarAbundances.cbegin()};
         }
 
         /**
@@ -809,8 +826,8 @@ namespace fourdst::composition {
          * for species are defined based on their atomic mass. When iterating over the molar abundance map, species will be
          * seen in order from lightest to heaviest.
          */
-        [[nodiscard]] std::map<atomic::Species, double>::iterator end() override {
-            return m_molarAbundances.end();
+        [[nodiscard]] detail::CompositionIterator<false> end() override {
+            return {m_species.end(), m_molarAbundances.end()};
         }
 
         /**
@@ -830,8 +847,8 @@ namespace fourdst::composition {
          * for species are defined based on their atomic mass. When iterating over the molar abundance map, species will be
          * seen in order from lightest to heaviest.
          */
-        [[nodiscard]] std::map<atomic::Species, double>::const_iterator end() const override {
-            return m_molarAbundances.cend();
+        [[nodiscard]] detail::CompositionIterator<true> end() const override {
+            return {m_species.cend(), m_molarAbundances.cend()};
         }
 
         [[nodiscard]] std::size_t hash() const override;
@@ -841,18 +858,9 @@ namespace fourdst::composition {
     inline bool operator==(const Composition& a, const Composition& b) noexcept {
         if (a.size() != b.size()) return false;
 
-        // Compare species sets quickly
         if (a.getRegisteredSpecies() != b.getRegisteredSpecies())
             return false;
 
-        // Compare all abundances
-        for (auto itA = a.begin(), itB = b.begin();
-             itA != a.end() && itB != b.end(); ++itA, ++itB) {
-            if (itA->first != itB->first)
-                return false;
-            if (itA->second != itB->second)
-                return false;
-             }
-        return true;
+        return a.hash() == b.hash();
     }
 }; // namespace fourdst::composition
diff --git a/src/composition/include/fourdst/composition/composition_abstract.h b/src/composition/include/fourdst/composition/composition_abstract.h
index 999561a..ab425d6 100644
--- a/src/composition/include/fourdst/composition/composition_abstract.h
+++ b/src/composition/include/fourdst/composition/composition_abstract.h
@@ -1,10 +1,10 @@
 #pragma once
 
 #include "fourdst/atomic/atomicSpecies.h"
+#include "fourdst/composition/iterators/composition_abstract_iterator.h"
 
 #include <string>
 #include <unordered_map>
-#include <map>
 #include <set>
 #include <vector>
 #include <memory>
@@ -35,6 +35,8 @@ namespace fourdst::composition {
      */
     class CompositionAbstract {
     public:
+        using iterator = detail::CompositionIterator<false>;
+        using const_iterator = detail::CompositionIterator<true>;
         /**
          * @brief Virtual destructor.
          */
@@ -66,7 +68,7 @@ namespace fourdst::composition {
          * @brief Get all registered atomic species in the composition.
          * @return A set of registered atomic species.
          */
-        [[nodiscard]] virtual const std::set<fourdst::atomic::Species> &getRegisteredSpecies() const noexcept = 0;
+        [[nodiscard]] virtual const std::vector<atomic::Species> &getRegisteredSpecies() const noexcept = 0;
 
         /**
          * @brief Get the mass fraction for all registered symbols.
@@ -175,10 +177,10 @@ namespace fourdst::composition {
 
         [[nodiscard]] virtual std::unique_ptr<CompositionAbstract> clone() const = 0;
 
-        [[nodiscard]] virtual std::map<atomic::Species, double>::iterator begin() = 0;
-        [[nodiscard]] virtual std::map<atomic::Species, double>::iterator end() = 0;
-        [[nodiscard]] virtual std::map<atomic::Species, double>::const_iterator begin() const = 0;
-        [[nodiscard]] virtual std::map<atomic::Species, double>::const_iterator end() const = 0;
+        [[nodiscard]] virtual iterator begin() = 0;
+        [[nodiscard]] virtual iterator end() = 0;
+        [[nodiscard]] virtual const_iterator begin() const = 0;
+        [[nodiscard]] virtual const_iterator end() const = 0;
 
         [[nodiscard]] virtual std::size_t hash() const = 0;
     };
diff --git a/src/composition/include/fourdst/composition/decorators/composition_decorator_abstract.h b/src/composition/include/fourdst/composition/decorators/composition_decorator_abstract.h
index 4eca926..8898377 100644
--- a/src/composition/include/fourdst/composition/decorators/composition_decorator_abstract.h
+++ b/src/composition/include/fourdst/composition/decorators/composition_decorator_abstract.h
@@ -1,6 +1,7 @@
 #pragma once
 
 #include "fourdst/atomic/atomicSpecies.h"
+#include "fourdst/composition/iterators/composition_abstract_iterator.h"
 
 #include "fourdst/composition/composition_abstract.h"
 
@@ -19,7 +20,7 @@ namespace fourdst::composition {
         [[nodiscard]] bool contains(const std::string& symbol) const override { return m_base_composition->contains(symbol); };
         [[nodiscard]] size_t size() const noexcept override { return m_base_composition->size(); };
         [[nodiscard]] std::set<std::string> getRegisteredSymbols() const noexcept override { return m_base_composition->getRegisteredSymbols(); };
-        [[nodiscard]] const std::set<atomic::Species> &getRegisteredSpecies() const noexcept override { return m_base_composition->getRegisteredSpecies(); };
+        [[nodiscard]] const std::vector<atomic::Species> &getRegisteredSpecies() const noexcept override { return m_base_composition->getRegisteredSpecies(); };
         [[nodiscard]] std::unordered_map<atomic::Species, double> getMassFraction() const noexcept override { return m_base_composition->getMassFraction(); };
         [[nodiscard]] std::unordered_map<atomic::Species, double> getNumberFraction() const noexcept override { return m_base_composition->getNumberFraction(); };
         [[nodiscard]] double getMassFraction(const std::string& symbol) const override { return m_base_composition->getMassFraction(symbol); };
@@ -36,13 +37,12 @@ namespace fourdst::composition {
         [[nodiscard]] size_t getSpeciesIndex(const std::string& symbol) const override { return m_base_composition->getSpeciesIndex(symbol); };
         [[nodiscard]] size_t getSpeciesIndex(const atomic::Species& species) const override { return m_base_composition->getSpeciesIndex(species); };
         [[nodiscard]] atomic::Species getSpeciesAtIndex(const size_t index) const override { return m_base_composition->getSpeciesAtIndex(index); }
-        [[nodiscard]] size_t hash() const override { return m_base_composition->hash(); };
 
-        [[nodiscard]] std::map<atomic::Species, double>::iterator begin() override { return m_base_composition->begin(); };
-        [[nodiscard]] std::map<atomic::Species, double>::iterator end() override { return m_base_composition->end(); };
+        [[nodiscard]] detail::CompositionIterator<false> begin() override { return m_base_composition->begin(); };
+        [[nodiscard]] detail::CompositionIterator<false> end() override { return m_base_composition->end(); };
 
-        [[nodiscard]] std::map<atomic::Species, double>::const_iterator begin() const override { return std::as_const(*m_base_composition).begin(); };
-        [[nodiscard]] std::map<atomic::Species, double>::const_iterator end() const override { return std::as_const(*m_base_composition).end(); };
+        [[nodiscard]] detail::CompositionIterator<true> begin() const override { return std::as_const(*m_base_composition).begin(); };
+        [[nodiscard]] detail::CompositionIterator<true> end() const override { return std::as_const(*m_base_composition).end(); };
     protected:
         std::unique_ptr<CompositionAbstract> m_base_composition;
     };
diff --git a/src/composition/include/fourdst/composition/decorators/composition_masked.h b/src/composition/include/fourdst/composition/decorators/composition_masked.h
index d9254ac..b7e275e 100644
--- a/src/composition/include/fourdst/composition/decorators/composition_masked.h
+++ b/src/composition/include/fourdst/composition/decorators/composition_masked.h
@@ -1,29 +1,36 @@
 #pragma once
 
+#include <vector>
+#include <set>
+#include <unordered_map>
+#include <memory>
+#include <string>
+
+#include "fourdst/composition/composition_abstract.h"
 #include "fourdst/composition/decorators/composition_decorator_abstract.h"
-#include "fourdst/composition/exceptions/exceptions_composition.h"
+#include "fourdst/composition/iterators/composition_abstract_iterator.h"
 #include "fourdst/atomic/atomicSpecies.h"
 
 namespace fourdst::composition {
     class MaskedComposition final : public CompositionDecorator {
     public:
+        using iterator = detail::CompositionIterator<false>;
+        using const_iterator = detail::CompositionIterator<true>;
+
         MaskedComposition(
             const CompositionAbstract& baseComposition,
-            const std::set<atomic::Species>& activeSpecies
+            const std::vector<atomic::Species>& activeSpecies
         );
 
         [[nodiscard]] bool contains(const atomic::Species &species) const noexcept override;
-
         [[nodiscard]] bool contains(const std::string &symbol) const override;
 
-        [[nodiscard]] const std::set<atomic::Species>& getRegisteredSpecies() const noexcept override;
-
+        [[nodiscard]] const std::vector<atomic::Species>& getRegisteredSpecies() const noexcept override;
         [[nodiscard]] std::set<std::string> getRegisteredSymbols() const noexcept override;
 
         [[nodiscard]] size_t size() const noexcept override;
 
         [[nodiscard]] std::unordered_map<atomic::Species, double> getMassFraction() const noexcept override;
-
         [[nodiscard]] std::unordered_map<atomic::Species, double> getNumberFraction() const noexcept override;
 
         [[nodiscard]] double getMassFraction(const std::string &symbol) const override;
@@ -37,29 +44,26 @@ namespace fourdst::composition {
         [[nodiscard]] double getElectronAbundance() const noexcept override;
 
         [[nodiscard]] std::vector<double> getMassFractionVector() const noexcept override;
-
         [[nodiscard]] std::vector<double> getNumberFractionVector() const noexcept override;
-
         [[nodiscard]] std::vector<double> getMolarAbundanceVector() const noexcept override;
 
         [[nodiscard]] size_t getSpeciesIndex(const std::string &symbol) const override;
-
         [[nodiscard]] size_t getSpeciesIndex(const atomic::Species &species) const override;
-
         [[nodiscard]] atomic::Species getSpeciesAtIndex(size_t index) const override;
 
         [[nodiscard]] std::unique_ptr<CompositionAbstract> clone() const override;
 
-        [[nodiscard]] std::map<atomic::Species, double>::iterator begin() override;
+        [[nodiscard]] iterator begin() override;
+        [[nodiscard]] iterator end() override;
 
-        [[nodiscard]] std::map<atomic::Species, double>::iterator end() override;
+        [[nodiscard]] const_iterator begin() const override;
+        [[nodiscard]] const_iterator end() const override;
 
-        [[nodiscard]] std::map<atomic::Species, double>::const_iterator begin() const override;
+        [[nodiscard]] size_t hash() const override;
 
-        [[nodiscard]] std::map<atomic::Species, double>::const_iterator end() const override;
     private:
-        std::set<atomic::Species> m_activeSpecies;
-        std::map<atomic::Species, double> m_masked_composition;
+        std::vector<atomic::Species> m_activeSpecies;
+        std::vector<double> m_molarAbundances;
     };
 
 }
\ No newline at end of file
diff --git a/src/composition/include/fourdst/composition/exceptions/exceptions_composition.h b/src/composition/include/fourdst/composition/exceptions/exceptions_composition.h
index 6e06015..5487265 100644
--- a/src/composition/include/fourdst/composition/exceptions/exceptions_composition.h
+++ b/src/composition/include/fourdst/composition/exceptions/exceptions_composition.h
@@ -2,6 +2,7 @@
 
 #include <exception>
 #include <string>
+#include <utility>
 
 namespace fourdst::composition::exceptions {
     /**
@@ -22,14 +23,14 @@ namespace fourdst::composition::exceptions {
          * @brief Constructs a CompositionError with an error message.
          * @param message The error message.
          */
-        explicit CompositionError(const std::string& message)
+        explicit CompositionError(std::string  message)
             : m_message(std::move(message)) {}
 
         /**
          * @brief Returns the error message.
          * @return A C-style string containing the error message.
          */
-        const char* what() const noexcept override{
+        [[nodiscard]] const char* what() const noexcept override{
             return m_message.c_str();
         }
     };
@@ -60,10 +61,10 @@ namespace fourdst::composition::exceptions {
     protected:
         std::string m_message;
     public:
-        explicit SpeciesError(const std::string& message)
+        explicit SpeciesError(std::string  message)
             : m_message(std::move(message)) {}
 
-        const char* what() const noexcept override {
+        [[nodiscard]] const char* what() const noexcept override {
             return m_message.c_str();
         }
     };
diff --git a/src/composition/include/fourdst/composition/iterators/composition_abstract_iterator.h b/src/composition/include/fourdst/composition/iterators/composition_abstract_iterator.h
new file mode 100644
index 0000000..a356376
--- /dev/null
+++ b/src/composition/include/fourdst/composition/iterators/composition_abstract_iterator.h
@@ -0,0 +1,106 @@
+#pragma once
+
+#include <vector>
+#include <iterator>
+#include <utility>
+#include <compare>
+
+#include "fourdst/atomic/atomicSpecies.h"
+
+namespace fourdst::composition::detail {
+
+    template <bool IsConst>
+    class CompositionIterator {
+    public:
+        using iterator_category = std::random_access_iterator_tag;
+        using difference_type   = std::ptrdiff_t;
+        // Returns a pair of references. Natively supports structured binding [sp, y]
+        using value_type        = std::pair<const atomic::Species, double>;
+
+
+        // Define reference types based on const-ness
+        using SpeciesRef = const atomic::Species&;
+        using AbundRef   = std::conditional_t<IsConst, const double&, double&>;
+        using reference  = std::pair<SpeciesRef, AbundRef>;
+
+        struct ArrowProxy {
+            reference m_payload;
+            const reference* operator->() const { return &m_payload; }
+        };
+
+        using pointer = ArrowProxy;
+
+    private:
+        using SpecIt = std::vector<atomic::Species>::const_iterator;
+        using AbunIt = std::conditional_t<IsConst,
+                                          std::vector<double>::const_iterator,
+                                          std::vector<double>::iterator>;
+
+        SpecIt m_sIt;
+        AbunIt m_aIt;
+
+    public:
+        CompositionIterator() = default;
+        CompositionIterator(SpecIt sIt, AbunIt aIt) : m_sIt(sIt), m_aIt(aIt) {}
+
+        template <bool WasConst, typename = std::enable_if_t<IsConst && !WasConst>>
+        CompositionIterator(const CompositionIterator<WasConst>& other)
+            : m_sIt(other.getSpeciesIt()), m_aIt(other.getAbundanceIt()) {}
+
+        [[nodiscard]] SpecIt getSpeciesIt() const { return m_sIt; }
+        [[nodiscard]] AbunIt getAbundanceIt() const { return m_aIt; }
+
+        reference operator*() const {
+            return { *m_sIt, *m_aIt };
+        }
+
+        ArrowProxy operator->() const {
+            return ArrowProxy{ **this };
+        }
+
+        reference operator[](difference_type n) const {
+            return { *(m_sIt + n), *(m_aIt + n) };
+        }
+
+        // --- Movement ---
+        CompositionIterator& operator++() { ++m_sIt; ++m_aIt; return *this; }
+        CompositionIterator operator++(int) { auto tmp = *this; ++(*this); return tmp; }
+
+        CompositionIterator& operator--() { --m_sIt; --m_aIt; return *this; } // FIXED
+        CompositionIterator operator--(int) { auto tmp = *this; --(*this); return tmp; }
+
+        CompositionIterator& operator+=(difference_type n) { m_sIt += n; m_aIt += n; return *this; }
+        CompositionIterator& operator-=(difference_type n) { m_sIt -= n; m_aIt -= n; return *this; }
+
+        // --- Arithmetic ---
+        friend CompositionIterator operator+(CompositionIterator it, difference_type n) { return it += n; }
+
+        // Commutative addition (n + it)
+        friend CompositionIterator operator+(difference_type n, CompositionIterator it) { return it += n; }
+        friend CompositionIterator operator-(CompositionIterator it, difference_type n) { return it -= n; }
+
+        // Difference between iterators
+        friend difference_type operator-(const CompositionIterator& lhs, const CompositionIterator& rhs) {
+            return lhs.m_sIt - rhs.m_sIt;
+        }
+
+        template <bool R>
+        bool operator==(const CompositionIterator<R>& other) const { return m_sIt == other.getSpeciesIt(); }
+
+        template <bool R>
+        bool operator!=(const CompositionIterator<R>& other) const { return m_sIt != other.getSpeciesIt(); }
+
+        template <bool R>
+        bool operator<(const CompositionIterator<R>& other) const { return m_sIt < other.getSpeciesIt(); }
+
+        template <bool R>
+        bool operator>(const CompositionIterator<R>& other) const { return m_sIt > other.getSpeciesIt(); }
+
+        template <bool R>
+        bool operator<=(const CompositionIterator<R>& other) const { return m_sIt <= other.getSpeciesIt(); }
+
+        template <bool R>
+        bool operator>=(const CompositionIterator<R>& other) const { return m_sIt >= other.getSpeciesIt(); }
+    };
+
+}
\ No newline at end of file
diff --git a/src/composition/include/fourdst/composition/utils/composition_hash.h b/src/composition/include/fourdst/composition/utils/composition_hash.h
index a9b3542..86c7ce8 100644
--- a/src/composition/include/fourdst/composition/utils/composition_hash.h
+++ b/src/composition/include/fourdst/composition/utils/composition_hash.h
@@ -1,7 +1,6 @@
 #pragma once
 
 #include <cstring>
-#include <cmath>
 #include <vector>
 #include <bit>
 
@@ -10,8 +9,12 @@
 #include "fourdst/composition/composition_abstract.h"
 
 namespace fourdst::composition::utils {
+    // Make a concept which checks if a type inherits from CompositionAbstract
+    template <typename CompT>
+    concept CompositionType = std::is_base_of_v<CompositionAbstract, CompT>;
+
     struct CompositionHash {
-        template <typename CompositionT>
+        template <CompositionType CompositionT>
         static uint64_t hash_exact(const CompositionT& comp) {
             uint64_t h0 = kSeed;
             uint64_t h1 = kSeed ^ kPrime1;
@@ -99,13 +102,19 @@ namespace fourdst::composition::utils {
     };
 }
 
-namespace std {
-    template<>
-    struct hash<fourdst::composition::Composition> {
-        std::size_t operator()(const fourdst::composition::Composition& c) const noexcept {
-            return static_cast<std::size_t>(
-                fourdst::composition::utils::CompositionHash::hash_exact(c)
-            );
-        }
-    };
-}
\ No newline at end of file
+template<>
+struct std::hash<fourdst::composition::CompositionAbstract> {
+    std::size_t operator()(const fourdst::composition::CompositionAbstract& c) const noexcept {
+        return static_cast<std::size_t>(
+            fourdst::composition::utils::CompositionHash::hash_exact(c)
+        );
+    }
+};
+template<>
+struct std::hash<fourdst::composition::Composition> {
+    std::size_t operator()(const fourdst::composition::Composition& c) const noexcept {
+        return static_cast<std::size_t>(
+            fourdst::composition::utils::CompositionHash::hash_exact(c)
+        );
+    }
+};
diff --git a/src/composition/lib/composition.cpp b/src/composition/lib/composition.cpp
index d19a4fa..1887ff5 100644
--- a/src/composition/lib/composition.cpp
+++ b/src/composition/lib/composition.cpp
@@ -34,36 +34,15 @@
 #include "fourdst/atomic/atomicSpecies.h"
 #include "fourdst/atomic/species.h"
 #include "fourdst/composition/composition.h"
+
+#include <numeric>
+
 #include "fourdst/composition/utils/composition_hash.h"
 #include "fourdst/composition/utils.h"
 
 #include "fourdst/composition/exceptions/exceptions_composition.h"
 
 namespace {
-    template<typename A, typename B>
-    std::vector<A> sortVectorBy(
-        std::vector<A> toSort,
-        const std::vector<B>& by
-    ) {
-        std::vector<std::size_t> indices(by.size());
-        for (size_t i = 0; i < indices.size(); i++) {
-            indices[i] = i;
-        }
-
-        std::ranges::sort(indices, [&](size_t a, size_t b) {
-            return by[a] < by[b];
-        });
-
-        std::vector<A> sorted;
-        sorted.reserve(indices.size());
-
-        for (const auto idx: indices) {
-            sorted.push_back(toSort[idx]);
-        }
-
-        return sorted;
-    }
-
     void throw_unknown_symbol(quill::Logger* logger, const std::string& symbol) {
         LOG_ERROR(logger, "Symbol {} is not a valid species symbol (not in the species database)", symbol);
         throw fourdst::composition::exceptions::UnknownSymbolError("Symbol " + symbol + " is not a valid species symbol (not in the species database)");
@@ -76,127 +55,142 @@ namespace {
 }
 
 namespace fourdst::composition {
-    Composition::Composition(
-    const std::vector<std::string>& symbols
-    ) {
-        for (const auto& symbol : symbols) {
-            registerSymbol(symbol);
-        }
-    }
+
+    /////////////////////////////////////////////
+    /// Constructors without molar abundances ///
+    /// These all delegate to the ctor        ///
+    /// vector<Species>                       ///
+    /////////////////////////////////////////////
 
     Composition::Composition(
         const std::set<std::string>& symbols
-    ) {
-        for (const auto& symbol : symbols) {
-            registerSymbol(symbol);
-        }
-    }
+    ) : Composition(symbols | std::ranges::to<std::vector>()) {}
+
+    Composition::Composition(
+        const std::set<atomic::Species> &species
+    ) : Composition(species | std::ranges::to<std::vector>()) {}
+
+    Composition::Composition(
+        const std::unordered_set<std::string> &symbols
+    ) : Composition(symbols | std::ranges::to<std::vector>()) {}
+
+    Composition::Composition(
+        const std::unordered_set<atomic::Species> &species
+    ) : Composition(species | std::ranges::to<std::vector>()) {}
+
+    Composition::Composition(
+        const std::vector<std::string>& symbols
+    ) : Composition(symbolVectorToSpeciesVector(symbols)) {}
 
     Composition::Composition(
         const std::vector<atomic::Species> &species
     ) {
-        for (const auto& s : species) {
-            registerSpecies(s);
-        }
+        m_species = species;
+        std::ranges::sort(m_species, [&](const atomic::Species& a, const atomic::Species& b) {
+            return a < b;
+        });
+
+        const auto last = std::ranges::unique(m_species).begin();
+        m_species.erase(last, m_species.end());
+
+        m_molarAbundances.resize(m_species.size(), 0.0);
     }
 
-    Composition::Composition(
-        const std::set<atomic::Species> &species
-    ) {
-        for (const auto& s : species) {
-            registerSpecies(s);
-        }
-    }
-
-    Composition::Composition(const std::unordered_set<std::string> &symbols) {
-        for (const auto& symbol : symbols) {
-            registerSymbol(symbol);
-        }
-    }
-
-    Composition::Composition(const std::unordered_set<atomic::Species> &species) {
-        for (const auto& s : species) {
-            registerSpecies(s);
-        }
-    }
+    //////////////////////////////////////////
+    /// Constructors with molar abundances ///
+    /// These all delegate to the ctor     ///
+    /// vector<Species, vector<double>>    ///
+    //////////////////////////////////////////
 
     Composition::Composition(
         const std::vector<std::string>& symbols,
         const std::vector<double>& molarAbundances
-    ) {
-        if (symbols.size() != molarAbundances.size()) {
-            LOG_CRITICAL(getLogger(), "The number of symbols and molarAbundances must be equal (got {} symbols and {} molarAbundances).", symbols.size(), molarAbundances.size());
-            throw exceptions::InvalidCompositionError("The number of symbols and fractions must be equal. Got " + std::to_string(symbols.size()) + " symbols and " + std::to_string(molarAbundances.size()) + " fractions.");
-        }
+    ) : Composition(symbolVectorToSpeciesVector(symbols), molarAbundances) {}
 
-        for (const auto &[symbol, y] : std::views::zip(symbols, molarAbundances)) {
-            registerSymbol(symbol);
-            setMolarAbundance(symbol, y);
-        }
-    }
+    Composition::Composition(
+        const std::set<std::string> &symbols,
+        const std::vector<double> &molarAbundances
+    ) : Composition(symbolVectorToSpeciesVector(symbols | std::ranges::to<std::vector>()), molarAbundances) {}
+
+
+    Composition::Composition(
+        const std::unordered_map<std::string, double> &symbolMolarAbundances
+    ) : Composition(
+        symbolMolarAbundances | std::views::keys | std::ranges::to<std::vector>(),
+        symbolMolarAbundances | std::views::values | std::ranges::to<std::vector>()
+        ) {}
+
+    Composition::Composition(
+        const std::map<std::string, double> &symbolMolarAbundances
+    ) : Composition(
+        symbolMolarAbundances | std::views::keys | std::ranges::to<std::vector>(),
+        symbolMolarAbundances | std::views::values | std::ranges::to<std::vector>()
+        ) {}
+
+    Composition::Composition(
+        const std::unordered_map<atomic::Species, double> &speciesMolarAbundances
+    ) : Composition(
+        speciesMolarAbundances | std::views::keys | std::ranges::to<std::vector>(),
+        speciesMolarAbundances | std::views::values | std::ranges::to<std::vector>()
+        ) {}
+
+    Composition::Composition(
+        const std::map<atomic::Species, double> &speciesMolarAbundances
+    ) : Composition(
+        speciesMolarAbundances | std::views::keys | std::ranges::to<std::vector>(),
+        speciesMolarAbundances | std::views::values | std::ranges::to<std::vector>()
+        ) {}
 
     Composition::Composition(
         const std::vector<atomic::Species> &species,
         const std::vector<double> &molarAbundances
     ) {
-        if (species.size() != molarAbundances.size()) {
+        if (__builtin_expect(species.size() != molarAbundances.size(), 0)) {
             LOG_CRITICAL(getLogger(), "The number of species and molarAbundances must be equal (got {} species and {} molarAbundances).", species.size(), molarAbundances.size());
             throw exceptions::InvalidCompositionError("The number of species and fractions must be equal. Got " + std::to_string(species.size()) + " species and " + std::to_string(molarAbundances.size()) + " fractions.");
         }
 
-        for (const auto& [s, y] : std::views::zip(species, molarAbundances)) {
-            registerSpecies(s);
-            setMolarAbundance(s, y);
+        const size_t numSpecies = species.size();
+        m_species.reserve(numSpecies);
+        m_molarAbundances.reserve(numSpecies);
+
+        for (size_t i = 0; i < numSpecies; ++i) {
+            m_species.push_back(species[i]);
+            if (__builtin_expect(molarAbundances[i] < 0.0, 0)) {
+                LOG_CRITICAL(getLogger(), "Molar abundance for species {} is negative (y = {}). Molar abundances must be non-negative.", species[i].name(), molarAbundances[i]);
+                throw exceptions::InvalidCompositionError("Molar abundance for species " + std::string(species[i].name()) + " is negative (y = " + std::to_string(molarAbundances[i]) + "). Molar abundances must be non-negative.");
+            }
+            m_molarAbundances.push_back(molarAbundances[i]);
         }
+
+        auto combined = std::views::zip(m_species, m_molarAbundances);
+
+        std::ranges::sort(combined, [](const auto& a, const auto& b) -> bool {
+            const auto& spA = std::get<0>(a);
+            const auto& spB = std::get<0>(b);
+
+            if (spA != spB) {
+                return spA < spB;
+            }
+
+            return std::get<1>(a) > std::get<1>(b);
+        });
+
+        auto [first, last] = std::ranges::unique(combined, [](const auto& a, const auto& b) {
+            return std::get<0>(a) == std::get<0>(b);
+        });
+
+        const auto newEndIndex = std::distance(combined.begin(), first);
+        m_species.erase(m_species.begin() + newEndIndex, m_species.end());
+        m_molarAbundances.erase(m_molarAbundances.begin() + newEndIndex, m_molarAbundances.end());
     }
 
-    Composition::Composition(
-        const std::set<std::string> &symbols,
-        const std::vector<double> &molarAbundances
-    ) {
-        if (symbols.size() != molarAbundances.size()) {
-            LOG_CRITICAL(getLogger(), "The number of symbols and molarAbundances must be equal (got {} symbols and {} molarAbundances).", symbols.size(), molarAbundances.size());
-            throw exceptions::InvalidCompositionError("The number of symbols and fractions must be equal. Got " + std::to_string(symbols.size()) + " symbols and " + std::to_string(molarAbundances.size()) + " fractions.");
-        }
+    ////////////////////////////////////////////
+    /// Copy and conversion constructors     ///
+    ////////////////////////////////////////////
 
-        for (const auto& [symbol, y] : std::views::zip(sortVectorBy<std::string>(std::vector<std::string>(symbols.begin(), symbols.end()), molarAbundances), molarAbundances)) {
-            registerSymbol(symbol);
-            setMolarAbundance(symbol, y);
-        }
-    }
-
-    Composition::Composition(const std::unordered_map<std::string, double> &symbolMolarAbundances) {
-        for (const auto& [symbol, y] : symbolMolarAbundances) {
-            registerSymbol(symbol);
-            setMolarAbundance(symbol, y);
-        }
-    }
-
-    Composition::Composition(const std::map<std::string, double> &symbolMolarAbundances) {
-        for (const auto& [symbol, y] : symbolMolarAbundances) {
-            registerSymbol(symbol);
-            setMolarAbundance(symbol, y);
-        }
-    }
-
-    Composition::Composition(const std::unordered_map<atomic::Species, double> &speciesMolarAbundances) {
-        for (const auto& [species, y] : speciesMolarAbundances) {
-            registerSpecies(species);
-            setMolarAbundance(species, y);
-        }
-    }
-
-    Composition::Composition(const std::map<atomic::Species, double> &speciesMolarAbundances) {
-        for (const auto& [species, y] : speciesMolarAbundances) {
-            registerSpecies(species);
-            setMolarAbundance(species, y);
-        }
-    }
-
-    Composition::Composition(
-        const Composition &composition
-    ) {
-        m_registeredSpecies = composition.m_registeredSpecies;
+    Composition::Composition(const Composition &composition) {
+        m_species = composition.m_species;
         m_molarAbundances = composition.m_molarAbundances;
     }
 
@@ -211,12 +205,32 @@ namespace fourdst::composition {
         const Composition &other
     ) {
         if (this != &other) {
-            m_registeredSpecies = other.m_registeredSpecies;
+            m_species = other.m_species;
             m_molarAbundances   = other.m_molarAbundances;
         }
+        m_cache.clear();
+        return *this;
+    }
+
+    Composition & Composition::operator=(const CompositionAbstract &other) {
+        m_species.clear();
+        m_molarAbundances.clear();
+        m_cache.clear();
+        for (const auto& species : other.getRegisteredSpecies()) {
+            registerSpecies(species);
+            setMolarAbundance(species, other.getMolarAbundance(species));
+        }
         return *this;
     }
 
+    std::unique_ptr<CompositionAbstract> Composition::clone() const {
+        return std::make_unique<Composition>(*this);
+    }
+
+    //------------------------------------------
+    // Registration methods
+    //------------------------------------------
+
     void Composition::registerSymbol(
         const std::string& symbol
     ) {
@@ -231,41 +245,190 @@ namespace fourdst::composition {
     void Composition::registerSymbol(
         const std::vector<std::string>& symbols
     ) {
-        for (const auto& symbol : symbols) {
-            registerSymbol(symbol);
-        }
+        registerSpecies(symbolVectorToSpeciesVector(symbols));
     }
 
     void Composition::registerSpecies(
         const atomic::Species &species
     ) noexcept {
-        m_registeredSpecies.insert(species);
-        if (!m_molarAbundances.contains(species)) {
-            m_molarAbundances.emplace(species, 0.0);
+        if (const auto it = std::ranges::lower_bound(m_species, species); it == m_species.end() || *it != species) {
+            const auto index = std::distance(m_species.begin(), it);
+            m_species.insert(it, species);
+            m_molarAbundances.insert(m_molarAbundances.begin() + index, 0.0);
+            m_cache.clear();
         }
     }
 
     void Composition::registerSpecies(
         const std::vector<atomic::Species> &species
     ) noexcept {
-        for (const auto& s : species) {
-            registerSpecies(s);
+        // We do not simply call registerSpecies(species) here as that would have a complexity of O(n^2) due to constantly
+        // reinserting into the vector. Rather we build the vector once and then sort it
+
+        if (species.empty()) return;
+
+        const size_t total_size = m_species.size() + species.size();
+        m_species.reserve(total_size);
+        m_molarAbundances.reserve(total_size);
+
+        for (const auto& sp : species) {
+            m_species.push_back(sp);
+            m_molarAbundances.push_back(0.0);
         }
+
+        auto combined = std::views::zip(m_species, m_molarAbundances);
+
+        std::ranges::sort(combined, [](const auto& a, const auto& b) {
+            const auto& speciesA = std::get<0>(a);
+            const auto& speciesB = std::get<0>(b);
+
+            if (speciesA != speciesB) {
+                return speciesA < speciesB;
+            }
+
+            return std::get<1>(a) > std::get<1>(b);
+        });
+
+        auto [first, last] = std::ranges::unique(combined, [](const auto& a, const auto& b) {
+            return std::get<0>(a) == std::get<0>(b);
+        });
+
+        const auto newEndIndex = std::distance(combined.begin(), first);
+
+        m_species.erase(m_species.begin() + newEndIndex, m_species.end());
+        m_molarAbundances.erase(m_molarAbundances.begin() + newEndIndex, m_molarAbundances.end());
+
+        m_cache.clear();
     }
 
     std::set<std::string> Composition::getRegisteredSymbols() const noexcept {
         std::set<std::string> symbols;
-        for (const auto& species : m_registeredSpecies) {
+        for (const auto& species : m_species) {
             symbols.insert(std::string(species.name()));
         }
         return symbols;
     }
 
-    const std::set<atomic::Species> &Composition::getRegisteredSpecies() const noexcept {
-        return m_registeredSpecies;
+    const std::vector<atomic::Species> &Composition::getRegisteredSpecies() const noexcept {
+        return m_species;
     }
 
 
+    //------------------------------------------
+    // Molar abundance setters
+    //------------------------------------------
+
+    void Composition::setMolarAbundance(
+        const std::string &symbol,
+        const double &molar_abundance
+    ) {
+        const auto species = getSpecies(symbol);
+        if (__builtin_expect(!species, 0)) {
+            throw_unknown_symbol(getLogger(), symbol);
+        }
+
+        setMolarAbundance(species.value(), molar_abundance);
+    }
+
+    void Composition::setMolarAbundance(
+        const atomic::Species &species,
+        const double &molar_abundance
+    ) {
+        if (__builtin_expect(molar_abundance < 0.0, 0)) {
+            LOG_ERROR(getLogger(), "Molar abundance must be non-negative for symbol {}. Currently it is {}.", species.name(), molar_abundance);
+            throw exceptions::InvalidCompositionError("Molar abundance must be non-negative, got " + std::to_string(molar_abundance) + " for symbol " + std::string(species.name()) + ".");
+        }
+
+        const std::expected<std::ptrdiff_t, SpeciesIndexLookupError> speciesIndexResult = findSpeciesIndex(species);
+        if (__builtin_expect(!speciesIndexResult, 0)) {
+            throw_unregistered_symbol(getLogger(), std::string(species.name()));
+        }
+
+        assert(speciesIndexResult.value() < m_molarAbundances.size());
+
+        m_molarAbundances[speciesIndexResult.value()] = molar_abundance;
+        m_cache.clear();
+    }
+
+
+    ////----------------------------------------------
+    ///   Methods which set multiple molar abundances
+    ///   delegate to vector<Species>, vector<double>
+    ///-----------------------------------------------
+
+    void Composition::setMolarAbundance(
+        const std::vector<std::string> &symbols,
+        const std::vector<double> &molar_abundances
+    ) {
+        setMolarAbundance(symbolVectorToSpeciesVector(symbols), molar_abundances);
+    }
+
+    void Composition::setMolarAbundance(
+        const std::set<std::string> &symbols,
+        const std::vector<double> &molar_abundances
+    ) {
+        setMolarAbundance(symbolVectorToSpeciesVector(symbols | std::ranges::to<std::vector>()), molar_abundances);
+    }
+
+    void Composition::setMolarAbundance(
+        const std::set<atomic::Species> &species,
+        const std::vector<double> &molar_abundances
+    ) {
+        setMolarAbundance(species | std::ranges::to<std::vector>(), molar_abundances);
+    }
+
+    void Composition::setMolarAbundance(
+        const std::vector<atomic::Species> &species,
+        const std::vector<double> &molar_abundances
+    ) {
+        if (__builtin_expect(species.size() != molar_abundances.size(), 0)) {
+            LOG_CRITICAL(getLogger(), "The number of species and molar_abundances must be equal (got {} species and {} molar_abundances).", species.size(), molar_abundances.size());
+            throw exceptions::InvalidCompositionError("The number of species and fractions must be equal. Got " + std::to_string(species.size()) + " species and " + std::to_string(molar_abundances.size()) + " fractions.");
+        }
+
+        if (species.empty()) return;
+
+        if (species.size() == m_species.size()) {
+            if (species == m_species) {
+                for (const auto& [sp, y] : std::views::zip(species, molar_abundances)) {
+                    if (__builtin_expect(y < 0.0, 0)) {
+                        LOG_ERROR(getLogger(), "Molar abundance must be non-negative. Instead got {} for species {}.", y, sp.name());
+                        throw exceptions::InvalidCompositionError("Molar abundance must be non-negative. Instead got " + std::to_string(y) + " for species " + std::string(sp.name()) + ".");
+                    }
+                }
+
+                m_molarAbundances = molar_abundances;
+                m_cache.clear();
+                return;
+            }
+        }
+
+        for (size_t i  = 0; i < species.size(); ++i) {
+            const double y = molar_abundances[i];
+            const auto& sp = species[i];
+            if (__builtin_expect(y < 0.0, 0)) {
+                LOG_CRITICAL(getLogger(), "Molar abundance must be non-negative. Instead got {} for species {}.", y, sp.name());
+                throw exceptions::InvalidCompositionError("Molar abundance must be non-negative. Instead got " + std::to_string(y) + " for species " + std::string(sp.name()) + ".");
+            }
+
+            const std::expected<std::ptrdiff_t, SpeciesIndexLookupError> speciesIndexResult = findSpeciesIndex(sp);
+            if (__builtin_expect(!speciesIndexResult, 0)) {
+                throw_unregistered_symbol(getLogger(), std::string(sp.name()));
+            }
+
+            const std::ptrdiff_t speciesIndex = speciesIndexResult.value();
+
+            m_molarAbundances[speciesIndex] = y;
+        }
+
+        m_cache.clear();
+    }
+
+
+    //------------------------------------------
+    // Fraction and abundance getters
+    //------------------------------------------
+
     double Composition::getMassFraction(const std::string& symbol) const {
         const auto species = getSpecies(symbol);
         if (!species) {
@@ -277,22 +440,26 @@ namespace fourdst::composition {
     double Composition::getMassFraction(
         const atomic::Species &species
     ) const {
-        if (!m_molarAbundances.contains(species)) {
+        const std::expected<std::ptrdiff_t, SpeciesIndexLookupError> speciesIndexResult = findSpeciesIndex(species);
+        if (!speciesIndexResult) {
             throw_unregistered_symbol(getLogger(), std::string(species.name()));
         }
-        std::map<atomic::Species, double> raw_mass;
+
         double totalMass = 0;
-        for (const auto& [sp, y] : m_molarAbundances) {
+        double speciesMass = 0;
+        for (const auto& [sp, y] : *this) {
             const double contrib = y * sp.mass();
             totalMass += contrib;
-            raw_mass.emplace(sp, contrib);
+            if (sp == species) {
+                speciesMass = contrib;
+            }
         }
-        return raw_mass.at(species) / totalMass;
+        return speciesMass / totalMass;
     }
 
     std::unordered_map<atomic::Species, double> Composition::getMassFraction() const noexcept {
         std::unordered_map<atomic::Species, double> mass_fractions;
-        for (const auto &species: m_molarAbundances | std::views::keys) {
+        for (const auto &species: *this | std::views::keys) {
             mass_fractions.emplace(species, getMassFraction(species));
         }
         return mass_fractions;
@@ -312,19 +479,23 @@ namespace fourdst::composition {
     double Composition::getNumberFraction(
         const atomic::Species &species
     ) const {
-        if (!m_molarAbundances.contains(species)) {
+        const std::expected<std::ptrdiff_t, SpeciesIndexLookupError> speciesIndexResult = findSpeciesIndex(species);
+        if (!speciesIndexResult) {
             throw_unregistered_symbol(getLogger(), std::string(species.name()));
         }
-        double total_moles_per_gram = 0.0;
-        for (const auto &y: m_molarAbundances | std::views::values) {
-            total_moles_per_gram += y;
-        }
-        return m_molarAbundances.at(species) / total_moles_per_gram;
+        const std::ptrdiff_t speciesIndex = speciesIndexResult.value();
+
+        const double total_moles_per_gram = std::accumulate(
+            m_molarAbundances.begin(),
+            m_molarAbundances.end(),
+            0.0
+        );
+        return m_molarAbundances[speciesIndex] / total_moles_per_gram;
     }
 
     std::unordered_map<atomic::Species, double> Composition::getNumberFraction() const noexcept {
         std::unordered_map<atomic::Species, double> number_fractions;
-        for (const auto &species: m_molarAbundances | std::views::keys) {
+        for (const auto &species: m_species) {
             number_fractions.emplace(species, getNumberFraction(species));
         }
         return number_fractions;
@@ -344,25 +515,34 @@ namespace fourdst::composition {
     double Composition::getMolarAbundance(
         const atomic::Species &species
     ) const {
-        if (!m_molarAbundances.contains(species)) {
+        const std::expected<std::ptrdiff_t, SpeciesIndexLookupError> speciesIndexResult = findSpeciesIndex(species);
+        if (!speciesIndexResult) {
             throw_unregistered_symbol(getLogger(), std::string(species.name()));
         }
-        return m_molarAbundances.at(species);
+        const std::ptrdiff_t speciesIndex = speciesIndexResult.value();
+        return m_molarAbundances[speciesIndex];
     }
 
+    //------------------------------------------
+    // Derived property getters
+    //------------------------------------------
+
     double Composition::getMeanParticleMass() const noexcept {
-        std::vector<double> X = getMassFractionVector();
-        double sum = 0.0;
-        for (const auto& [species, x] : std::views::zip(m_registeredSpecies, X)) {
-            sum += x/species.mass();
+        double totalMass = 0.0;
+        double totalMoles = 0.0;
+
+        for (size_t i = 0; i < m_species.size(); ++i) {
+            totalMoles += m_molarAbundances[i];
+            totalMass  += m_molarAbundances[i] * m_species[i].mass();
         }
 
-        return 1.0 / sum;
+        return totalMass / totalMoles;
     }
 
+
     double Composition::getElectronAbundance() const noexcept {
         double Ye = 0.0;
-        for (const auto& [species, y] : m_molarAbundances) {
+        for (const auto& [species, y] : *this) {
             Ye += species.z() * y;
         }
         return Ye;
@@ -377,8 +557,8 @@ namespace fourdst::composition {
             return m_cache.canonicalComp.value(); // Short circuit if we have cached the canonical composition
         }
         CanonicalComposition canonicalComposition;
-        const std::set<Species> canonicalH = {H_1, H_2, H_3, H_4, H_5, H_6, H_7};
-        const std::set<Species> canonicalHe = {He_3, He_4, He_5, He_6, He_7, He_8, He_9, He_10};
+        static const std::unordered_set<Species> canonicalH = {H_1, H_2, H_3, H_4, H_5, H_6, H_7};
+        static const std::unordered_set<Species> canonicalHe = {He_3, He_4, He_5, He_6, He_7, He_8, He_9, He_10};
 
         for (const auto& symbol : canonicalH) {
             if (contains(symbol)) {
@@ -391,11 +571,8 @@ namespace fourdst::composition {
             }
         }
 
-        for (const auto& species : m_molarAbundances | std::views::keys) {
-            const bool isHIsotope = canonicalH.contains(species);
-            const bool isHeIsotope = canonicalHe.contains(species);
-
-            if (isHIsotope || isHeIsotope) {
+        for (const auto& species : m_species) {
+            if (canonicalH.contains(species) || canonicalHe.contains(species)) {
                 continue; // Skip canonical H and He symbols
             }
 
@@ -412,25 +589,25 @@ namespace fourdst::composition {
         return canonicalComposition;
     }
 
+    //------------------------------------------
+    // Vector getters
+    //------------------------------------------
+
     std::vector<double> Composition::getMassFractionVector() const noexcept {
         if (m_cache.massFractions.has_value()) {
             return m_cache.massFractions.value(); // Short circuit if we have cached the mass fractions
         }
 
         std::vector<double> massFractionVector;
-        std::vector<double> speciesMass;
 
         massFractionVector.reserve(m_molarAbundances.size());
-        speciesMass.reserve(m_molarAbundances.size());
 
-        for (const auto &species: m_molarAbundances | std::views::keys) {
+        for (const auto &species: m_species) {
             massFractionVector.push_back(getMassFraction(species));
-            speciesMass.push_back(species.mass());
         }
 
-        std::vector<double> massFractions = sortVectorBy(massFractionVector, speciesMass);
-        m_cache.massFractions = massFractions; // Cache the result
-        return massFractions;
+        m_cache.massFractions = massFractionVector; // Cache the result
+        return massFractionVector;
 
     }
 
@@ -440,43 +617,25 @@ namespace fourdst::composition {
         }
 
         std::vector<double> numberFractionVector;
-        std::vector<double> speciesMass;
 
         numberFractionVector.reserve(m_molarAbundances.size());
-        speciesMass.reserve(m_molarAbundances.size());
 
-        for (const auto &species: m_molarAbundances | std::views::keys) {
+        for (const auto &species: m_species) {
             numberFractionVector.push_back(getNumberFraction(species));
-            speciesMass.push_back(species.mass());
         }
 
-        std::vector<double> numberFractions = sortVectorBy(numberFractionVector, speciesMass);
-        m_cache.numberFractions = numberFractions; // Cache the result
-        return numberFractions;
+        m_cache.numberFractions = numberFractionVector; // Cache the result
+        return numberFractionVector;
     }
 
     std::vector<double> Composition::getMolarAbundanceVector() const noexcept {
-        if (m_cache.molarAbundances.has_value()) {
-            return m_cache.molarAbundances.value(); // Short circuit if we have cached the molar abundances
-        }
-
-        std::vector<double> molarAbundanceVector;
-        std::vector<double> speciesMass;
-
-        molarAbundanceVector.reserve(m_molarAbundances.size());
-        speciesMass.reserve(m_molarAbundances.size());
-
-        for (const auto &[species, y]: m_molarAbundances) {
-            molarAbundanceVector.push_back(y);
-            speciesMass.push_back(species.mass());
-        }
-
-        std::vector<double> molarAbundances = sortVectorBy(molarAbundanceVector, speciesMass);
-        m_cache.molarAbundances = molarAbundances; // Cache the result
-        return molarAbundances;
-
+        return m_molarAbundances;
     }
 
+    //------------------------------------------
+    // Species index getters and lookups
+    //------------------------------------------
+
     size_t Composition::getSpeciesIndex(
         const std::string &symbol
     ) const {
@@ -491,66 +650,36 @@ namespace fourdst::composition {
     size_t Composition::getSpeciesIndex(
         const atomic::Species &species
     ) const {
-        if (!m_registeredSpecies.contains(species)) {
-            LOG_ERROR(getLogger(), "Species {} is not in the composition.", species.name());
-            throw exceptions::UnregisteredSymbolError("Species " + std::string(species.name()) + " is not in the composition.");
-        }
-        if (m_cache.sortedSpecies.has_value()) {
-            return std::distance(
-                m_cache.sortedSpecies->begin(),
-                std::ranges::find(
-                    m_cache.sortedSpecies.value().begin(),
-                    m_cache.sortedSpecies.value().end(),
-                    species
-                )
-            );
+        std::expected<std::ptrdiff_t, SpeciesIndexLookupError> speciesIndexResult = findSpeciesIndex(species);
+        if (!speciesIndexResult) {
+            switch (speciesIndexResult.error()) {
+                case SpeciesIndexLookupError::NO_REGISTERED_SPECIES:
+                    [[fallthrough]];
+                case SpeciesIndexLookupError::SPECIES_NOT_FOUND:
+                    throw_unregistered_symbol(getLogger(), std::string(species.name()));
+                default:
+                    throw std::logic_error("Unhandled SpeciesIndexLookupError in Composition::getSpeciesIndex");
+            }
         }
 
-        std::vector<atomic::Species> speciesVector;
-        std::vector<double> speciesMass;
-
-        speciesVector.reserve(m_molarAbundances.size());
-        speciesMass.reserve(m_molarAbundances.size());
-
-        for (const auto &s: m_registeredSpecies) {
-            speciesVector.emplace_back(s);
-            speciesMass.push_back(s.mass());
-        }
-
-        std::vector<atomic::Species> sortedSpecies = sortVectorBy(speciesVector, speciesMass);
-        m_cache.sortedSpecies = sortedSpecies;
-        return std::distance(sortedSpecies.begin(), std::ranges::find(sortedSpecies, species));
+        return static_cast<size_t>(speciesIndexResult.value());
     }
 
     atomic::Species Composition::getSpeciesAtIndex(
         const size_t index
     ) const {
-        if (m_cache.sortedSpecies.has_value()) {
-            return m_cache.sortedSpecies.value().at(index);
+        if (index >= m_species.size()) {
+            LOG_ERROR(getLogger(), "Index {} is out of bounds for registered species (size {}).", index, m_species.size());
+            throw std::out_of_range("Index " + std::to_string(index) + " is out of bounds for registered species (size " + std::to_string(m_species.size()) + ").");
         }
 
-        std::vector<atomic::Species> speciesVector;
-        std::vector<double> speciesMass;
-
-        speciesVector.reserve(m_molarAbundances.size());
-        speciesMass.reserve(m_molarAbundances.size());
-
-        for (const auto &species: m_registeredSpecies) {
-            speciesVector.emplace_back(species);
-            speciesMass.push_back(species.mass());
-        }
-
-        std::vector<atomic::Species> sortedSymbols = sortVectorBy(speciesVector, speciesMass);
-        if (index >= sortedSymbols.size()) {
-            LOG_ERROR(getLogger(), "Index {} is out of range for composition of size {}.", index, sortedSymbols.size());
-            throw std::out_of_range("Index " + std::to_string(index) + " is out of range for composition of size " + std::to_string(sortedSymbols.size()) + ".");
-        }
-        return sortedSymbols.at(index);
+        return m_species[index];
     }
 
-    std::unique_ptr<CompositionAbstract> Composition::clone() const {
-        return std::make_unique<Composition>(*this);
-    }
+
+    //------------------------------------------
+    // Utility methods
+    //------------------------------------------
 
     std::size_t Composition::hash() const {
         if (m_cache.hash.has_value()) {
@@ -564,7 +693,7 @@ namespace fourdst::composition {
     bool Composition::contains(
         const atomic::Species &species
     ) const noexcept {
-        return m_registeredSpecies.contains(species);
+        return std::ranges::binary_search(m_species, species);
     }
 
     bool Composition::contains(
@@ -578,74 +707,41 @@ namespace fourdst::composition {
     }
 
     size_t Composition::size() const noexcept {
-        return m_registeredSpecies.size();
+        return m_species.size();
     }
 
-    void Composition::setMolarAbundance(
-        const std::string &symbol,
-        const double &molar_abundance
-    ) {
-        const auto species = getSpecies(symbol);
-        if (!species) {
-            throw_unknown_symbol(getLogger(), symbol);
+    std::expected<std::ptrdiff_t, Composition::SpeciesIndexLookupError> Composition::findSpeciesIndex(const atomic::Species &species) const noexcept {
+        if (m_species.empty()) return std::unexpected(SpeciesIndexLookupError::NO_REGISTERED_SPECIES);
+
+        const auto it = std::ranges::lower_bound(m_species, species);
+
+        if (it == m_species.end() || *it != species) {
+            return std::unexpected(SpeciesIndexLookupError::SPECIES_NOT_FOUND);
         }
 
-        setMolarAbundance(species.value(), molar_abundance);
+        return std::distance(m_species.begin(), it);
     }
 
-    void Composition::setMolarAbundance(
-        const atomic::Species &species,
-        const double &molar_abundance
-    ) {
-        if (!m_registeredSpecies.contains(species)) {
-            throw_unregistered_symbol(getLogger(), std::string(species.name()));
-        }
-        if (molar_abundance < 0.0) {
-            LOG_ERROR(getLogger(), "Molar abundance must be non-negative for symbol {}. Currently it is {}.", species.name(), molar_abundance);
-            throw exceptions::InvalidCompositionError("Molar abundance must be non-negative, got " + std::to_string(molar_abundance) + " for symbol " + std::string(species.name()) + ".");
+    std::vector<atomic::Species> Composition::symbolVectorToSpeciesVector(const std::vector<std::string> &symbols) {
+        std::vector<atomic::Species> species;
+        species.reserve(symbols.size());
+
+
+        for (const auto& symbol : symbols) {
+            const auto speciesResult = getSpecies(symbol);
+            if (!speciesResult) {
+                throw_unknown_symbol(getLogger(), symbol);
+            }
+            species.push_back(speciesResult.value());
         }
 
-        m_cache.clear();
-        m_molarAbundances.at(species) = molar_abundance;
+        return species;
     }
 
-    void Composition::setMolarAbundance(
-        const std::vector<std::string> &symbols,
-        const std::vector<double> &molar_abundances
-    ) {
-        for (const auto& [symbol, y] : std::views::zip(symbols, molar_abundances)) {
-            setMolarAbundance(symbol, y);
-        }
-    }
 
-    void Composition::setMolarAbundance(
-        const std::vector<atomic::Species> &species,
-        const std::vector<double> &molar_abundances
-    ) {
-        for (const auto& [s, y] : std::views::zip(species, molar_abundances)) {
-            setMolarAbundance(s, y);
-        }
-    }
-
-    void Composition::setMolarAbundance(
-        const std::set<std::string> &symbols,
-        const std::vector<double> &molar_abundances
-    ) {
-        for (const auto& [symbol, y] : std::views::zip(symbols, molar_abundances)) {
-            setMolarAbundance(symbol, y);
-        }
-    }
-
-    void Composition::setMolarAbundance(
-        const std::set<atomic::Species> &species,
-        const std::vector<double> &molar_abundances
-    ) {
-        for (const auto& [s, y] : std::views::zip(species, molar_abundances)) {
-            setMolarAbundance(s, y);
-        }
-    }
-
-    /// OVERLOADS
+    //------------------------------------------
+    // Stream operator
+    //------------------------------------------
 
     std::ostream& operator<<(
         std::ostream& os,
@@ -653,7 +749,7 @@ namespace fourdst::composition {
     ) {
         os << "Composition(Mass Fractions => [";
         size_t count = 0;
-        for (const auto &species : composition.m_registeredSpecies) {
+        for (const auto &species : composition.m_species) {
             os << species << ": " << composition.getMassFraction(species);
             if (count < composition.size() - 1) {
                 os << ", ";
diff --git a/src/composition/lib/decorators/composition_masked.cpp b/src/composition/lib/decorators/composition_masked.cpp
index 1edbd11..0052077 100644
--- a/src/composition/lib/decorators/composition_masked.cpp
+++ b/src/composition/lib/decorators/composition_masked.cpp
@@ -1,29 +1,41 @@
 #include "fourdst/composition/decorators/composition_masked.h"
-
+#include "fourdst/composition/exceptions/exceptions_composition.h"
 #include "fourdst/atomic/species.h"
+
+#include <algorithm>
 #include <memory>
+#include <string>
+#include <vector>
+#include <set>
+#include <unordered_map>
+
+#include "fourdst/composition/utils/composition_hash.h"
 
 namespace fourdst::composition {
-MaskedComposition::MaskedComposition(
+    MaskedComposition::MaskedComposition(
         const CompositionAbstract& baseComposition,
-        const std::set<atomic::Species>& activeSpecies
+        const std::vector<atomic::Species>& activeSpecies
     ) :
     CompositionDecorator(baseComposition.clone()),
     m_activeSpecies(activeSpecies) {
+
+        std::ranges::sort(m_activeSpecies, [](const auto &a, const auto &b) {
+            return a < b;
+        });
+
+        m_molarAbundances.reserve(m_activeSpecies.size());
         for (const auto& species : m_activeSpecies) {
-            if (CompositionDecorator::contains(species)) {
-                m_masked_composition.emplace(species, CompositionDecorator::getMolarAbundance(species));
+            if (!CompositionDecorator::contains(species)) {
+                m_molarAbundances.push_back(0.0);
             } else {
-                m_masked_composition.emplace(species, 0.0);
+                m_molarAbundances.push_back(CompositionDecorator::getMolarAbundance(species));
             }
         }
+
     }
 
     bool MaskedComposition::contains(const atomic::Species &species) const noexcept{
-        if (m_activeSpecies.contains(species)) {
-            return true;
-        }
-        return false;
+        return std::ranges::contains(m_activeSpecies, species);
     }
 
     bool MaskedComposition::contains(const std::string &symbol) const {
@@ -31,14 +43,10 @@ MaskedComposition::MaskedComposition(
             throw exceptions::UnknownSymbolError("Cannot find species '" + symbol + "' in base composition");
         }
         const atomic::Species& species = atomic::species.at(symbol);
-        if (m_activeSpecies.contains(species)) {
-            return true;
-        }
-
-        return false;
+        return contains(species);
     }
 
-    const std::set<atomic::Species>& MaskedComposition::getRegisteredSpecies() const noexcept {
+    const std::vector<atomic::Species>& MaskedComposition::getRegisteredSpecies() const noexcept {
         return m_activeSpecies;
     }
 
@@ -179,11 +187,19 @@ MaskedComposition::MaskedComposition(
         if (!contains(symbol)) {
             throw exceptions::UnregisteredSymbolError("Species '" + symbol + "' is not part of the active species in the MaskedComposition.");
         }
-        return std::distance(m_activeSpecies.begin(), m_activeSpecies.find(atomic::species.at(symbol)));
+        return std::distance(
+            m_activeSpecies.begin(),
+            std::ranges::find_if(m_activeSpecies,
+            [&symbol](const atomic::Species& sp) {
+                    return std::string(sp.name()) == symbol;
+        }));
     }
 
     size_t MaskedComposition::getSpeciesIndex(const atomic::Species &species) const {
-        return std::distance(m_activeSpecies.begin(), m_activeSpecies.find(species));
+        return std::distance(
+            m_activeSpecies.begin(),
+            std::ranges::find(m_activeSpecies, species)
+        );
     }
 
     atomic::Species MaskedComposition::getSpeciesAtIndex(const size_t index) const {
@@ -199,19 +215,23 @@ MaskedComposition::MaskedComposition(
         return std::make_unique<MaskedComposition>(*m_base_composition, m_activeSpecies);
     }
 
-    std::map<atomic::Species, double>::iterator MaskedComposition::begin() {
-        return m_masked_composition.begin();
+    MaskedComposition::iterator MaskedComposition::begin() {
+        return {m_activeSpecies.begin(), m_molarAbundances.begin()};
     }
 
-    std::map<atomic::Species, double>::iterator MaskedComposition::end() {
-        return m_masked_composition.end();
+    MaskedComposition::iterator MaskedComposition::end() {
+        return  {m_activeSpecies.end(), m_molarAbundances.end()};
     }
 
-    std::map<atomic::Species, double>::const_iterator MaskedComposition::begin() const {
-        return m_masked_composition.cbegin();
+    MaskedComposition::const_iterator MaskedComposition::begin() const {
+        return {m_activeSpecies.cbegin(), m_molarAbundances.cbegin()};
     }
 
-    std::map<atomic::Species, double>::const_iterator MaskedComposition::end() const {
-        return m_masked_composition.cend();
+    MaskedComposition::const_iterator MaskedComposition::end() const {
+        return {m_activeSpecies.cend(), m_molarAbundances.cend()};
+    }
+
+    size_t MaskedComposition::hash() const {
+        return utils::CompositionHash::hash_exact<MaskedComposition>(*this);
     }
 };
diff --git a/tests/composition/compositionTest.cpp b/tests/composition/compositionTest.cpp
index 6e962b5..75fa64e 100644
--- a/tests/composition/compositionTest.cpp
+++ b/tests/composition/compositionTest.cpp
@@ -3,6 +3,7 @@
 #include <string>
 #include <algorithm>
 #include <chrono>
+#include <ranges>
 
 #include "fourdst/atomic/atomicSpecies.h"
 #include "fourdst/atomic/species.h"
@@ -176,12 +177,9 @@ TEST_F(compositionTest, setGetComposition) {
 TEST_F(compositionTest, getRegisteredSpecies) {
     fourdst::composition::Composition comp;
     comp.registerSpecies({fourdst::atomic::Be_7, fourdst::atomic::H_1, fourdst::atomic::He_4});
-    auto registeredSpecies = comp.getRegisteredSpecies();
-    EXPECT_TRUE(registeredSpecies.contains(fourdst::atomic::H_1));
-    EXPECT_TRUE(registeredSpecies.contains(fourdst::atomic::He_4));
-    EXPECT_FALSE(registeredSpecies.contains(fourdst::atomic::Li_6));
-    const auto it1 = registeredSpecies.begin();
-    EXPECT_EQ(*it1, fourdst::atomic::H_1);
+    EXPECT_TRUE(comp.contains(fourdst::atomic::H_1));
+    EXPECT_TRUE(comp.contains(fourdst::atomic::He_4));
+    EXPECT_FALSE(comp.contains(fourdst::atomic::Li_6));
 }
 
 /**
@@ -397,7 +395,7 @@ TEST_F(compositionTest, canonicalizeNaNIfAllowed) {
     fourdst::composition::Composition a, b;
     a.registerSymbol("H-1"); b.registerSymbol("H-1");
     double qnan1 = std::numeric_limits<double>::quiet_NaN();
-    double qnan2 = std::bit_cast<double>(std::uint64_t{0x7ff80000'00000042ULL});
+    auto qnan2 = std::bit_cast<double>(std::uint64_t{0x7ff80000'00000042ULL});
     a.setMolarAbundance("H-1", qnan1);
     b.setMolarAbundance("H-1", qnan2);
     ASSERT_EQ(fourdst::composition::utils::CompositionHash::hash_exact(a),
@@ -437,4 +435,51 @@ TEST_F(compositionTest, hashStaleing) {
     a.setMolarAbundance("C-12", 0.002);
     const std::size_t hash2 = a.hash();
     EXPECT_NE(hash1, hash2);
+}
+
+TEST_F(compositionTest, speciesOrdering) {
+    using namespace fourdst::atomic;
+    const std::unordered_map<Species, double> abundances ={
+        {C_12, 0.001},
+        {H_1, 0.702},
+        {O_16, 0.22},
+        {N_14, 0.0005},
+        {He_4, 0.06},
+        {Ar_30, 0.0001},
+        {Li_6, 0.0002}
+    };
+
+    const fourdst::composition::Composition a(abundances);
+    static const std::vector<Species> speciesInOrder = {
+        H_1, He_4, Li_6, C_12, N_14, O_16, Ar_30
+    };
+
+    const std::vector<Species>& specieFromComposition = a.getRegisteredSpecies();
+    for (const auto& [spe, spo] : std::views::zip(speciesInOrder, specieFromComposition)) {
+        EXPECT_EQ(spe, spo);
+    }
+}
+
+TEST_F(compositionTest, iterationOrdering) {
+    using namespace fourdst::atomic;
+    const std::unordered_map<Species, double> abundances ={
+        {C_12, 0.001},
+        {H_1, 0.702},
+        {O_16, 0.22},
+        {N_14, 0.0005},
+        {He_4, 0.06},
+        {Ar_30, 0.0001},
+        {Li_6, 0.0002}
+    };
+
+    const fourdst::composition::Composition a(abundances);
+    static const std::vector<Species> speciesInOrder = {
+        H_1, He_4, Li_6, C_12, N_14, O_16, Ar_30
+    };
+
+    size_t count = 0;
+    for (const auto &sp: a | std::views::keys) {
+        EXPECT_EQ(sp, speciesInOrder[count]);
+        ++count;
+    }
 }
\ No newline at end of file