where ̵ h is the reduced Planck's constant (J s), N A is Avogadro's number, c is the speed of light (m/s), m e is the mass of an electron (Kg), M is the molecular mass (amu). β is the electric dipole-magnetic dipole polarizability tensor, which is expressed in atomic units together with the frequency ω. The tensor can be written with a sum-over-states (SOS) formula as: