Constants

// ******************************************************************************
//
// Title:       Force Field X.
// Description: Force Field X - Software for Molecular Biophysics.
// Copyright:   Copyright (c) Michael J. Schnieders 2001-2025.
//
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package ffx.utilities;

import static org.apache.commons.math3.util.FastMath.PI;

/**
 * Library class containing constants such as Avogadro's number.
 * <p>
 * SI units: kg, m, s, C, K, mol, lm
 * <p>
 * Our typical units: g/mol, Angstrom, psec, elementary charges (+1 proton charge), K, mol, N/A
 *
 * @author Jacob M. Litman
 * @author Michael J. Schnieders
 * @since 1.0
 */
public class Constants {

  /**
   * The default constructor is private.
   */
  private Constants() {
    // Empty.
  }

  // The first seven constants the defining constants of SI as of May 20, 2019 (BIPM).
  /**
   * Hyperfine transition frequency of cesium in Hertz, defining the second.
   * <p>
   * <code>DEL_V_Cs_SI = 9192631770L</code>
   */
  public static final long DEL_V_Cs_SI = 9192631770L;
  /**
   * Speed of light in m/s, defining the meter.
   * <p>
   * <code>SPEED_OF_LIGHT_SI = 299792458</code>
   */
  public static final int SPEED_OF_LIGHT_SI = 299792458;
  /**
   * Planck constant in J*s, defining the kilogram (by defining the derived Joule).
   * <p>
   * <code>PLANCK_CONSTANT_SI = 6.62607015E-34d</code>
   */
  public static final double PLANCK_CONSTANT_SI = 6.62607015E-34d;
  /**
   * Elementary charge in Coulombs, defining the Coulomb.
   * <p>
   * <code>ELEMENTARY_CHARGE_SI = 1.602176634E-19d</code>
   */
  public static final double ELEMENTARY_CHARGE_SI = 1.602176634E-19d;
  /**
   * Boltzmann's constant in J/K, defining the Kelvin.
   * <p>
   * <code>BOLTZMANN_SI = 1.380649E-23d</code>
   */
  public static final double BOLTZMANN_SI = 1.380649E-23d;
  /**
   * Avogadro's number, defining the mol.
   * <p>
   * <code>AVOGADRO = 6.02214076E23d</code>
   */
  public static final double AVOGADRO = 6.02214076E23d;
  /**
   * Luminous efficacy in lm/W, defining the lumen.
   * <p>
   * <code>K_CD_SI = 683</code>
   */
  public static final int K_CD_SI = 683;
  /**
   * Constant <code>LITERS_PER_CUBIC_ANGSTROM=1E-27</code>
   */
  public static final double LITERS_PER_CUBIC_ANGSTROM = 1E-27;
  /**
   * Constant <code>ATM_TO_BAR=1.01325</code>
   */
  public static final double ATM_TO_BAR = 1.01325;
  /**
   * Constant <code>KCAL_TO_KJ=4.184</code>
   */
  public static final double KCAL_TO_KJ = 4.184;
  /**
   * Constant <code>KJ_TO_KCAL=1.0 / KCAL_TO_KJ</code>
   */
  public static final double KJ_TO_KCAL = 1.0 / KCAL_TO_KJ;
  /**
   * Constant <code>METERS_TO_ANG=1E10</code>
   */
  public static final double METERS_TO_ANG = 1E10;
  /**
   * Constant <code>NM_TO_ANG=10</code>
   */
  public static final double NM_TO_ANG = 10;
  /**
   * Constant <code>ANG_TO_NM=0.1</code>
   */
  public static final double ANG_TO_NM = 0.1;
  /**
   * Constant <code>SEC_TO_PSEC=1E12</code>
   */
  public static final double SEC_TO_PSEC = 1E12;
  /**
   * Constant <code>KG_TO_GRAMS=1000</code>
   */
  public static final double KG_TO_GRAMS = 1000;
  /**
   * Constant <code>PSEC_TO_FSEC=1000</code>
   */
  public static final double PSEC_TO_FSEC = 1000;
  /**
   * Constant <code>FSEC_TO_PSEC=0.001</code>
   */
  public static final double FSEC_TO_PSEC = 0.001;
  /**
   * Ideal gas constant in kcal/(mol*K)
   * <code>R = BOLTZMANN_SI * AVOGADRO * 0.001 * KJ_TO_KCAL</code>
   * <code>R = 0.0019872042586408316</code>
   */
  public static final double R = BOLTZMANN_SI * AVOGADRO * 0.001 * KJ_TO_KCAL;
  /**
   * Boltzmann/ideal gas constant in units of g*Ang^2/(mol*psec^2*K).
   * <code> kB = BOLTZMANN_SI * KG_TO_GRAMS * METERS_TO_ANG * METERS_TO_ANG * AVOGADRO / (SEC_TO_PSEC * SEC_TO_PSEC)</code>
   * <code>kB = 0.831446261815324</code>
   */
  public static final double kB = BOLTZMANN_SI * KG_TO_GRAMS * METERS_TO_ANG * METERS_TO_ANG * AVOGADRO / (SEC_TO_PSEC * SEC_TO_PSEC);
  /**
   * Conversion from kcal/mol/Ang^3 to Atm. <code>PRESCON=6.85684112e4</code>
   */
  public static final double PRESCON = 6.85684112e4;
  /**
   * Permittivity of water at STP. <code>dWater=78.3</code>
   */
  public static final double dWater = 78.3;
  /**
   * Convert nanoseconds to seconds. <code>NS2SEC=1e-9</code>
   */
  public static final double NS2SEC = 1e-9;
  /**
   * Room temperature ~= 298.15 Kelvins. <code>ROOM_TEMPERATURE=298.15</code>
   */
  public static final double ROOM_TEMPERATURE = 298.15;
  /**
   * Coulomb constant in units of kcal*Ang/(mol*electron^2), as derived from CODATA 2018 permittivity
   * of free space measured at 8.8541878128*10^-12 F/m <code>
   * ELECTRIC_CODATA_2018=332.063713299</code>
   */
  public static final double ELECTRIC_CODATA_2018 = 332.063713299;
  /**
   * Coulomb constant in units of kcal*Ang/(mol*electron^2)
   *
   * <p>Note -- this value varies slightly between force field definitions and can be set using the
   * ELECTRIC property. As such, it's not updated to SI/CODATA standards, but rather kept up-to-date
   * with the coulomb parameter in Tinker/source/units.f. At present, the Tinker value is a truncated
   * version of the Coulomb constant derived from CODATA 2018.<code>
   * DEFAULT_ELECTRIC=332.0637133</code>
   */
  public static final double DEFAULT_ELECTRIC = 332.0637133;
  /**
   * Conversion from electron-Angstroms to Debye. <code>ELEC_ANG_TO_DEBYE=4.80321</code>
   */
  public static final double ELEC_ANG_TO_DEBYE = 4.80321;
  /**
   * Conversion from electron-Angstroms^2 to Buckinghams. <code> ELEC_ANG2_TO_BUCKINGHAMS =
   * ELEC_ANG_TO_DEBYE * ELEC_ANG_TO_DEBYE</code>
   */
  public static final double ELEC_ANG2_TO_BUCKINGHAMS = ELEC_ANG_TO_DEBYE * ELEC_ANG_TO_DEBYE;
  /**
   * Conversion from kcal/mole to g*Ang**2/ps**2. <code>KCAL_TO_GRAM_ANG2_PER_PS2=4.1840e2</code>
   */
  public static final double KCAL_TO_GRAM_ANG2_PER_PS2 = 4.1840e2;
  /**
   * Conversion from Bohr to Angstroms. <code>BOHR=0.529177210903</code>
   */
  public static final double BOHR = 0.529177210903;
  /**
   * Conversion from Bohr^2 to Angstroms^2. <code>BOHR2 = BOHR * BOHR</code>
   */
  public static final double BOHR2 = BOHR * BOHR;
  /**
   * Convert Hartree to kcal/mol.
   */
  public static final double HARTREE_TO_KCAL_PER_MOL = 627.5094738898777;
  /**
   * Degrees per radian.
   */
  public static final double DEGREES_PER_RADIAN = 180.0 / PI;
}