where N A is the Avogadro's constant, e is the elementary charge, m e is the electron mass, c is the speed of light, ϵ 0 is the vacuum permittivity, n is the refractive index (here set to 1), f i is the calculated oscillator strength of the ith transition, ω i is the ith transition angular frequency, and and the line-broadening function g, typically a Lorentzian described with a γ i parameter corresponding to the half-width at half-maximum (HWHM).