GEOS-DEV · rrsettgast · Nov 27, 2025 · Jan 8, 2026 · Jan 9, 2026
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
@@ -2,6 +2,7 @@
 set( hpcReact_headers
      common/macros.hpp
      common/CArrayWrapper.hpp
+     constitutive/activity/Bdot.hpp
      reactions/exampleSystems/BulkGeneric.hpp
      reactions/geochemistry/Carbonate.hpp
      reactions/geochemistry/Forge.hpp
@@ -67,10 +68,11 @@ message(STATUS "HPCReact/src CMAKE_CURRENT_SOURCE_DIR: ${CMAKE_CURRENT_SOURCE_DI
 # hpcReact_add_code_checks( PREFIX hpcReact
 #                        EXCLUDES "blt/*" )
 
+add_subdirectory( common/unitTests )
+add_subdirectory( constitutive/unitTests)
 add_subdirectory( reactions/exampleSystems/unitTests )
 add_subdirectory( reactions/geochemistry/unitTests )
 add_subdirectory( reactions/massActions/unitTests )
-add_subdirectory( common/unitTests )
 add_subdirectory( docs )
 
 hpcReact_add_code_checks( PREFIX hpcReact

diff --git a/src/constitutive/activity/Bdot.hpp b/src/constitutive/activity/Bdot.hpp
@@ -0,0 +1,85 @@
+/*
+ * ------------------------------------------------------------------------------------------------------------
+ * SPDX-License-Identifier: (BSD-3-Clause)
+ *
+ * Copyright (c) 2025- Lawrence Livermore National Security LLC
+ * All rights reserved
+ *
+ * See top level LICENSE files for details.
+ * ------------------------------------------------------------------------------------------------------------
+ */
+#pragma once
+
+#include "DebyeHuckel.hpp"
+
+namespace hpcReact
+{
+
+template< typename REAL_TYPE,
+          typename INDEX_TYPE,
+          typename IONIC_STRENGTH_TYPE >
+class Bdot
+{
+public:
+  using RealType = REAL_TYPE;
+  using IndexType = INDEX_TYPE;
+
+
+
+struct Params : public IONIC_STRENGTH_TYPE::Params
+{
+  RealType m_ionSizeParameter[IONIC_STRENGTH_TYPE::Params::numSpecies()];
+  RealType m_bdotParameter[IONIC_STRENGTH_TYPE::Params::numSpecies()];
+};
+
+
+
+template< typename ARRAY_1D_TO_CONST,
+          typename ARRAY_1D >
+static inline HPCREACT_HOST_DEVICE 
+void
+calculateActivities( Params const & params,
+                     ARRAY_1D_TO_CONST const & speciesConcentrations,
+                     ARRAY_1D & activities )
+{
+
+  RealType const ionicStrength = IONIC_STRENGTH_TYPE::calculate( params, speciesConcentrations );
+  RealType const sqrtI = sqrt(ionicStrength);
+  RealType const rho_w = 997.0479; // kg/m3
+  RealType const eps_r = 78.54;    // dimensionless
+  RealType const T_K = 298.15;
+  RealType const A_gamma = DebyeHuckel<RealType>::A_gamma( T_K, rho_w, eps_r );
+  RealType const B_gamma = DebyeHuckel<RealType>::B_gamma( T_K, rho_w, eps_r );
+  auto const & speciesCharge = params.m_speciesCharge;
+  auto const & a = params.m_ionSizeParameter;
+  auto const & b = params.m_bdotParameter;
+
+
+
+  const IndexType numSpecies = params.numSpecies();
+  for( IndexType i=0; i<numSpecies; ++i )
+  {
+    RealType const DebyeHuckel_term = DebyeHuckel<RealType>::log10_gamma( sqrtI, 
+                                                                 speciesCharge[i], 
+                                                                 a[i], 
+                                                                 A_gamma,
+                                                                 B_gamma );
+    activities[i] = DebyeHuckel_term + b[i] * ionicStrength;
+  }
+}
+
+private:
+  // -------------------------------------------------------------
+  // Physical constants (CODATA 2018-ish), SI units
+  // -------------------------------------------------------------
+  static constexpr double pi   = 3.14159265358979323846;
+  static constexpr double e0   = 8.8541878128e-12;      // vacuum permittivity [F/m]
+  static constexpr double eChg = 1.602176634e-19;       // elementary charge [C]
+  static constexpr double kB   = 1.380649e-23;          // Boltzmann constant [J/K]
+  static constexpr double NA   = 6.02214076e23;         // Avogadro [1/mol]
+  static constexpr double ln10 = 2.3025850929940459;
+
+};
+
+
+} // namespace hpcReact
diff --git a/src/constitutive/activity/DebyeHuckel.hpp b/src/constitutive/activity/DebyeHuckel.hpp
@@ -0,0 +1,155 @@
+#pragma once
+
+#include "common/macros.hpp"
+
+#include <cmath>
+
+/**
+ * @file DebyeHuckel.hpp
+ * @brief Debye–Hückel A^γ and B parameters for aqueous electrolytes.
+ *
+ * This header provides helper functions to compute the Debye–Hückel
+ * parameters A^γ and B in their "native" (natural-log) form for
+ * molal (mol/kg) activity-coefficient models.
+ *
+ * The functions are expressed in terms of fundamental physical constants
+ * and water properties (density and relative permittivity). They can be
+ * used directly in Debye–Hückel or extended Debye–Hückel/B-dot models.
+ */
+
+template< typename REAL_TYPE >
+class DebyeHuckel
+{
+public:
+  using RealType = REAL_TYPE;
+
+  /// π (pi).
+  static constexpr RealType pi        = 3.141592653589793e+00;
+
+  /// Vacuum permittivity ε₀ [F/m].
+  static constexpr RealType e0        = 8.854187812800001e-12;
+
+  /// Elementary charge e [C].
+  static constexpr RealType eChg      = 1.602176634000000e-19;
+
+  /// Boltzmann constant k_B [J/K].
+  static constexpr RealType kB        = 1.380649000000000e-23;
+
+  /// Avogadro constant N_A [1/mol].
+  static constexpr RealType NA        = 6.022140760000000e+23;
+
+  /// Natural logarithm of 10, ln(10).
+  static constexpr RealType ln10      = 2.302585092994046e+00;
+
+  /// Inverse of ln(10).
+  static constexpr RealType invln10  = 4.342944819032518e-01;
+
+
+  // -------------------------------------------------------------
+  // Debye–Hückel A^γ (natural log, molal scale)
+  // -------------------------------------------------------------
+
+  /**
+   * @brief Debye–Hückel A^γ parameter in natural-log form.
+   *
+   * Computes the coefficient A^γ(T,ρ,ε_r) used in the Debye–Hückel
+   * expression for the natural logarithm of the activity coefficient:
+   *
+   * \f[
+   *  \ln \gamma_i =
+   *    - A^\gamma_{\ln}(T,P) \, z_i^2
+   *      \frac{\sqrt{I}}{1 + B(T,P)\, a_i \sqrt{I}}
+   * \f]
+   *
+   * where:
+   *   - \f$ I \f$ is ionic strength in mol/kg (molal),
+   *   - \f$ z_i \f$ is the ionic charge,
+   *   - \f$ a_i \f$ is the ion-size parameter (length),
+   *   - A^γ is independent of the log base (this function is for ln).
+   *
+   * The implementation follows the "native" Debye–Hückel form,
+   * using fundamental physical constants without any 1/ln(10) factors.
+   *
+   * @param T_K          Temperature in kelvin [K].
+   * @param rho_w  Density of water in g/L (≈ kg/m³ numerically).
+   * @param eps_r        Relative permittivity (dielectric constant) of water.
+   * @return A^γ in units consistent with molal ionic strength, for use
+   *         in ln(γ) expressions.
+   */
+  static inline HPCREACT_HOST_DEVICE 
+  RealType A_gamma( RealType const T_K,
+                        RealType const rho_w,
+                        RealType const eps_r )
+  {
+    RealType const num = ::pow( eChg, 3.0 ) * ::sqrt( 2.0 * pi * NA * rho_w );
+    RealType const den = ::pow( 4.0 * pi * e0 * eps_r * kB * T_K, 1.5 );
+    return num / den;
+  }
+
+
+  // -------------------------------------------------------------
+  // Debye–Hückel B (natural log, molal scale)
+  // -------------------------------------------------------------
+
+  /**
+   * @brief Debye–Hückel B parameter in natural-log form.
+   *
+   * Computes the Debye–Hückel length-scale parameter B(T,ρ,ε_r) used
+   * in the extended Debye–Hückel law:
+   *
+   * \f[
+   *  \ln \gamma_i =
+   *    - A^\gamma_{\ln}(T,P) \, z_i^2
+   *      \frac{\sqrt{I}}{1 + B(T,P)\, a_i \sqrt{I}} \; ,
+   * \f]
+   *
+   * where:
+   *   - \f$ I \f$ is ionic strength in mol/kg,
+   *   - \f$ a_i \f$ is an ion-size parameter (length).
+   *
+   * The combination \f$ B a_i \sqrt{I} \f$ is dimensionless; the
+   * absolute units of B therefore depend on the length units chosen
+   * for \f$ a_i \f$.
+   *
+   * @param T_K          Temperature in kelvin [K].
+   * @param rho_w  Density of water in g/L (≈ kg/m³ numerically).
+   * @param eps_r        Relative permittivity (dielectric constant) of water.
+   * @return B parameter for use in ln(γ) expressions.
+   */
+  static inline HPCREACT_HOST_DEVICE
+  RealType B_gamma( RealType const T_K,
+                  RealType const rho_w,
+                  RealType const eps_r )
+  {
+    RealType const num = 2.0 * NA * rho_w * eChg * eChg;
+    RealType const den = e0 * eps_r * kB * T_K;
+    return ::sqrt( num / den );
+  }
+
+
+  static inline HPCREACT_HOST_DEVICE
+  RealType log10_gamma( RealType const I,
+                        RealType const zi,
+                        RealType const ai,
+                        RealType const T_K )
+  {
+    RealType const sqrtI   = sqrt(I);
+    RealType const A       = A_gamma(T_K);
+    RealType const B       = B_gamma(T_K);
+    RealType const denom   = 1 + B * ai * sqrtI;
+    return -A * zi * zi * sqrtI / denom;
+  }
+
+
+  static inline HPCREACT_HOST_DEVICE
+  RealType log10_gamma( RealType const sqrtI,
+                        RealType const zi,
+                        RealType const ai,
+                        RealType const A,
+                        RealType const B )
+  {
+    RealType const denom   = 1 + B * ai * sqrtI;
+    return -A * zi * zi * sqrtI / denom;
+  }
+
+};
diff --git a/src/constitutive/activity/Identity.hpp b/src/constitutive/activity/Identity.hpp
@@ -0,0 +1,51 @@
+/*
+ * ------------------------------------------------------------------------------------------------------------
+ * SPDX-License-Identifier: (BSD-3-Clause)
+ *
+ * Copyright (c) 2025- Lawrence Livermore National Security LLC
+ * All rights reserved
+ *
+ * See top level LICENSE files for details.
+ * ------------------------------------------------------------------------------------------------------------
+ */
+#pragma once
+
+#include "common/macros.hpp"
+
+namespace hpcReact
+{
+
+template< typename REAL_TYPE,
+          typename INDEX_TYPE,
+          typename IONIC_STRENGTH_TYPE >
+class Identity
+{
+public:
+  using RealType = REAL_TYPE;
+  using IndexType = INDEX_TYPE;
+
+  struct Params
+  { 
+    HPCREACT_HOST_DEVICE static constexpr IndexType numSpecies() { return IONIC_STRENGTH_TYPE::Params::numSpecies(); }
+  };
+
+  template< typename ARRAY_1D_TO_CONST,
+            typename ARRAY_1D,
+            typename PARAMS >
+  static inline HPCREACT_HOST_DEVICE
+  void
+  calculateActivities( PARAMS const & ,
+                       ARRAY_1D_TO_CONST const & speciesConcentrations,
+                       ARRAY_1D & activities )
+  {
+    constexpr IndexType numSpecies = PARAMS::numSpecies();
+    for( IndexType i=0; i<numSpecies; ++i )
+    {
+      activities[i] = speciesConcentrations[i];
+    }
+  }
+
+};
+
+
+} // namespace hpcReact
diff --git a/src/constitutive/ionicStrength/SpeciatedIonicStrength.hpp b/src/constitutive/ionicStrength/SpeciatedIonicStrength.hpp
@@ -0,0 +1,59 @@
+/*
+ * ------------------------------------------------------------------------------------------------------------
+ * SPDX-License-Identifier: (BSD-3-Clause)
+ *
+ * Copyright (c) 2025- Lawrence Livermore National Security LLC
+ * All rights reserved
+ *
+ * See top level LICENSE files for details.
+ * ------------------------------------------------------------------------------------------------------------
+ */
+#pragma once
+
+#include "common/CArrayWrapper.hpp"
+#include "common/macros.hpp"
+
+namespace hpcReact
+{
+
+template< typename REAL_TYPE,
+          typename INDEX_TYPE,
+          int NUM_SPECIES >
+class SpeciatedIonicStrength
+{
+public:
+  using RealType = REAL_TYPE;
+  using IndexType = INDEX_TYPE;
+
+struct Params
+{
+
+  HPCREACT_HOST_DEVICE static constexpr IndexType numSpecies() { return NUM_SPECIES; }
+  HPCREACT_HOST_DEVICE constexpr CArrayWrapper< RealType, NUM_SPECIES > & speciesCharge() { return m_speciesCharge; }
+
+  CArrayWrapper< RealType, NUM_SPECIES > m_speciesCharge;
+};
+
+
+template< typename ARRAY_1D_TO_CONST >
+static inline HPCREACT_HOST_DEVICE 
+REAL_TYPE
+calculate( Params const & params,
+           ARRAY_1D_TO_CONST const & speciesConcentration )
+{
+  REAL_TYPE I = 0.0;
+  auto const &  numSpecies = params.numSpecies();
+  auto const & speciesCharge = params.m_speciesCharge;
+  for( int i=0; i<numSpecies; ++i )
+  {
+    I += speciesConcentration[i] * speciesCharge[i] * speciesCharge[i];
+  }
+  I *= 0.5;
+  return I;
+}
+
+
+};
+
+
+} // namespace hpcReact