OPTIMIZED EFFECTIVE-POTENTIAL CALCULATIONS OF GE AND GAAS

We compare pseudopotential calculations of the total energies, cohesiv e energies, and energy gaps of Ge and GaAs using the local-density app roximation (LDA) for exchange and correlation (re), a Hartree-Fock cor e, LDA valence re functional, and the optimized effective potential (O EP). We make a careful study of the errors introduced by our first-pri nciples pseudopotentials, which although still small, are larger than they were for Si. Conclusions drawn from Si and a preliminary Ge calcu lation remain valid. These are that the OEP greatly enhances some ener gy gaps and moderately enhances the rest, and that the LDA correlation functional, which is added on to the OEP exchange, always results in a semiconductor cohesive energy that is much too small.