units.h

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#ifndef RIZER_UNITS_H
#define RIZER_UNITS_H

 
#include <cmath>
 
namespace rizer {
namespace units {
 
// --- Fundamental constants (CODATA, https://physics.nist.gov/cuu) ----------

constexpr double e = 1.602'176'634e-19;
constexpr double k_b = 1.380'649e-23;
constexpr double N_a = 6.022'140'76e23;
constexpr double c = 299'792'458.0;
constexpr double h = 6.626'070'15e-34;
constexpr double m_e = 9.109'383'713'9e-31;
constexpr double M_e = m_e * N_a;
constexpr double Da = 1.660'539'068'92e-27;
constexpr double epsilon_0 = 8.854'187'818'8e-12;
constexpr double stefan_boltzmann = 5.670'374'419e-8;
constexpr double bohr_radius_m = 5.291'772'105'44e-11;
constexpr double rydberg_energy_eV = 13.605'693'122'990;

constexpr double pi = 3.141'592'653'589'793'238'46;
constexpr double hbar = h / (2.0 * pi);
 
// --- Derived constants ------------------------------------------------------

constexpr double R = k_b * N_a;
constexpr double R_kmol = R * 1e3;
constexpr double m_N2 = 28.013'4 * Da;
constexpr double P_1_bar = 1e5;
 
// --- Unit conversions -------------------------------------------------------

constexpr double eV_to_K = e / k_b;
constexpr double K_to_eV = k_b / e;
constexpr double eV_to_J = e;
constexpr double J_to_eV = 1.0 / e;
constexpr double K_to_J = k_b;
constexpr double J_to_K = 1.0 / k_b;
constexpr double kJ_per_mol_to_J_per_particle = 1e3 / N_a;
constexpr double invcm_to_joule = h * c * 100.0;

inline const double spitzer_constant =
    std::pow(4.0 * pi * epsilon_0 / e, 2.0) * std::pow(k_b, 1.5)
    / (4.0 / 3.0 * std::sqrt(2.0 * pi * m_e)) * 1.96;
 
// Note: units.py's `element_radius` dict is intentionally not mirrored here --
// the C++ collision model receives per-species radii through SpeciesSpec, so
// no compiled-in radius table is needed.
 
} // namespace units
} // namespace rizer
 
#endif // RIZER_UNITS_H