DIELECTRIC RESPONSE IN INSULATORS - A WAVE-VECTOR-DEPENDENT AND FREQUENCY-DEPENDENT MODEL

We have calculated a random-phase-approximation-type expression for th e dielectric-response function of several large-band-gap insulators us ing a development of the model suggested by Fry. This model should be good for systems with well-localized valence-electron functions and fl at valence-electron bands. We show that in this model it is a minor ap proximation to use spherically symmetric bands in the integration over reciprocal space. This allows simple and fast evaluations of our expr essions, thus avoiding Brillouin-zone integrations and summations over band indices. The results of our calculations are compared with ab in itio data on static dielectric matrices from Baldereschi and Tosatti, with model calculations on the static dielectric function from Rezvani and Friauf, and with electron-energy-loss spectra for LiF from Fields et al., using only one fit parameter. In addition the Johnson f-sum r ule is tested for this model. We also verify that our approximation gi ves the correct singular behavior of the (0, K) matrix elements.