""" 
    Physical constants
    ==================
"""
# speed of light
const c = 299792458
# elementary charge
const q = 1.6021766208e-19
# atomic mass unit
const u = 1.66053904e-27
# electron mass
const m_e = 9.10938356e-31
# Plank constant
const h = 6.62607004e-34
# Bohr radius
const a0 = 5.2917721067e-11
# Rydberg constant
const Ry = 10973731.6
# Boltzman constant
const kB = 1.38064852e-23