Using ab initio calculations based on the first principles along with EM theory, various radiative properties such as dielectric function (electrical permittivity, ε e,ω), spectral absorption coefficient (σ ph,ω), and the complex refraction index (m ω), are calculated for various interactions between photons and electric/magnetic entities in matter. For example, the imaginary part (ε e,c,ω) of complex dielectric function (ε e,ω = ε e,r,ω + i ε e,c,ω) for electronic transition across a bandgap is ε ε e , c , ω ω = 4 π π 2 ω ω 2 V ∑ ∑ i ∈ ∈ V B , j ∈ ∈ C B ∑ ∑ κ κ w κ κ | p i j | 2 δ δ ( E κ κ , j − − E κ κ , i − − ℏ ℏ ω ω ) , where V is the unit-cell volume, VB and CB denote the valence and conduction bands, w κ is the weight associated with a κ -point, and p ij is the transition momentum matrix element. The real part is ε e,r,ω is obtained from ε e,c,ω using the Kramers-Kronig relation ε ε e , r , ω ω = 1 + 4 π π P ∫ ∫ 0 ∞ ∞ d ω ω ′ ω ω ′ ε ε e , c , ω ω ′ ω ω ′ 2 − − ω ω 2 . Here, P denotes the principal value of the integral.