bohr = 4.0 * pi * epsilon_0 * hbar**2 / (electron_mass * elementary_charge**2) = a0
atomic_time = hbar / hartree = t0
kcalpermol = kcal / 6.0221412927e23
kjpermol = kJ / 6.0221412927e23
gpermol = g / 6.0221412927e23
fs = femtosecond = 1.0 * femtosecond
ps = picosecond = 1.0 * piscosecond
amu = 1.0 * atomic_mass_units
ang = 1.0 * angstrom
eV = electron_volt
debye = 0.393430307 * elementary_charge * bohr
