Optimized effective-potential calculations of Ge and GaAs

We compare pseudopotential calculations of the total energies, cohesive energies, and energy gaps of Ge and GaAs using the local-density approximation (LDA) for exchange and correlation (re), a Hartree-Fock core, LDA valence re functional, and the optimized effective potential (OEP). We make a careful study of the errors introduced by our first-principles pseudopotentials, which although still small, are larger than they were for Si. Conclusions drawn from Si and a preliminary Ge calculation remain valid. These are that the OEP greatly enhances some energy 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.