A periodic density functional theory and Hartree-Fock study of alkali halides with Gaussian orbitals

Periodic-density functional theory (DFT) and Hartree-Fock calculations have been performed on the alkali halides for the alkali metals Li, Na, K, and Rb and the halogens F, Cl, Br and I. Calculations were done with HF-optimiz ed basis sets and DFT-optimized basis sets, at the HF, local DFT (LDFT), an d nonlocal DFT (NLDFT) with the Becke exchange potential and the Perdew-Wan g correlation potential. The HF and NLDFT results are similar for the latti ce parameters, in general, by overestimating them. The LDFT method always u nderestimates the lattice parameters. As a consequence, the HF and NLDFT me thods predict bulk moduli that are too small and the LDFT method predicts b ulk moduli that are too large. The absolute error in the bulk moduli are la rger at the LDFT level than at the other levels. Charge distribution maps h ave been calculated and analyzed. The maps clearly show that the main diffe rence between the HF and DFT densities is in the treatment of exchange and that the nonlocal (gradient) corrections moderate this difference.