# Physical Constants

Physical constants in Psi4
Label | Value | Description
--- | --- | ---
psi_h | 6.62606896E-34 | The Planck constant (Js)
psi_c | 2.99792458E8 | Speed of light (ms$^{-1}$)
psi_kb | 1.3806504E-23 | The Boltzmann constant (JK$^{-1}$)
psi_R | 8.314472 | Universal gas constant (JK$^{-1}$mol$^{-1}$)
psi_bohr2angstroms | 0.52917720859 | Bohr to Angstroms conversion factor
psi_bohr2m | 0.52917720859E-10 | Bohr to meters conversion factor
psi_bohr2cm | 0.52917720859E-8 | Bohr to centimeters conversion factor
psi_amu2g | 1.660538782E-24 | Atomic mass units to grams conversion factor
psi_amu2kg | 1.660538782E-27 | Atomic mass units to kg conversion factor
psi_au2amu | 5.485799097E-4 | Atomic units (m$@@e$) to atomic mass units conversion factor
psi_hartree2J | 4.359744E-18 | Hartree to joule conversion factor
psi_hartree2aJ | 4.359744 | Hartree to attojoule (10$^{-18}$J) conversion factor
psi_cal2J | 4.184 | Calorie to joule conversion factor
psi_dipmom_au2si | 8.47835281E-30 | Atomic units to SI units (Cm) conversion factor for dipoles
psi_dipmom_au2debye | 2.54174623 | Atomic units to Debye conversion factor for dipoles
psi_dipmom_debye2si | 3.335640952E-30 | Debye to SI units (Cm) conversion factor for dipoles
psi_c_au | 137.035999679 | Speed of light in atomic units
psi_hartree2ev | 27.21138 | Hartree to eV conversion factor
psi_hartree2wavenumbers | 219474.6 | Hartree to cm$^{-1}$ conversion factor
psi_hartree2kcalmol | 627.5095 | Hartree to kcal mol$^{-1}$ conversion factor
psi_hartree2MHz | 6.579684E9 | Hartree to MHz conversion factor
psi_kcalmol2wavenumbers | 349.7551 | kcal mol$^{-1}$ to cm$^{-1}$ conversion factor
psi_e0 | 8.854187817E-12 | Vacuum permittivity (Fm$^{-1}$)
psi_na | 6.02214179E23 | Avagadro’s number
psi_me | 9.10938215E-31 | Electron rest mass (in kg)