In another example, for the far IR regions where the optical phonons are involved, the dielectric function (ε e,ω) are calculated as ε ε e , ω ω ε ε e , ∞ ∞ = 1 + ∑ ∑ j ω ω L O , j 2 − − ω ω T O , j 2 ω ω T O , j 2 − − ω ω 2 − − i γ γ ω ω , where LO and TO denote the longitudinal and transverse optical phonon modes, j is all the IR-active modes, and γ is the temperature-dependent damping term in the oscillator model. ε e,∞ is high frequency dielectric permittivity, which can be calculated DFT calculation when ions are treated as external potential.