Dielectric tensor, effective charges, and phonons in alpha -quartz by variational density-functional perturbation theory.

Abstract

A new method is introduced to calculate response functions within Density Functional Theory. It uses a conjugate-gradient algorithm applied to a variational expression from perturbation theory. The dielectric tensor, Born effective charges, and TO and LO phonons at q=0 are obtained. A one-parameter scissors operator gives the dielectric tensor within 0.5% of the experimental value. The anisotropy of the effective charge tensor is shown to be crucial for reproducing the LO-TO splittings.