Local Fields and Exchange-Correlation Effects in The Static Screening Response of Diamond, Si, Ge and LiCl

The density functional approach with ab initio pseudopotentials has been used to calculate the static dielectric matrices for Diamond, Si, Ge and LiCl form first principles for a regular grid of q-points in the Brillouin zone. These are required to calculate the response of the crystal to localized perturbations. Two types of perturbation are considered: a constant applied external electric field and an added point charge at various locations in the unit cell. These examples illustrate the local fields in the screening and the role of exchange and correlation (included in the local density approximation). Local fields are shown to be quantitatively important and to alter the qualitative features of the screening response.