A new strategy of getting accurate refractive index dispersion curves using first-principles sum-over-states method jointed by density functional perturbation theory

This paper introduces a new method for obtaining refractive index dispersion curves: the joint method of density-functional perturbation theory and first-principles sum-over-states. Since the refractive index dispersion curves and birefringence of LiNbO₃, LiGaSe₂ and BPO₄ materials computed by the first-principles sum-over-states method have some deviations, the density-functional perturbation theory method is introduced. However, the density-functional perturbation theory method can only obtain the refractive index at the optical frequency, so the first-principles sum-over-states combined with the density-functional perturbation theory method can be used to obtain the accurate refractive index dispersion curve. To further investigate the effect of parameters on the refractive index and birefringence, the different functionals, different cutoff energies, different k-points, ionic contributions and fixed symmetry are introduced for LiNbO₃, LiGaSe₂ and BPO₄ materials. The results show that symmetry-fixed nature of sum-over-states method is an important factor affecting the accuracy of the calculated birefringence.