Bulk and (100) surface d -> d excitation energies in NiO from first-principles Hartree-Fock calculations

First-principles, periodic, spin-unrestricted Hartree-Fock calculations are reported of bulk and (100) surface d-->d excitation energies in NiO from direct total energy differences. Comparisons are made with optical absorption and electron energy-loss spectra and with previous multi-reference CEPA and CASSCF/CASPT2 calculations for (NiO6)(10-) and (NiO5)(8-) clusters. It is shown that, as a result of their local character, the energies of these excitations are determined essentially by the coordination number of the excited atom rather than the nature of the local environment. From comparisons with the experimental data it is concluded that, for periodic/extended systems, the contribution from electron correlation is approximately 0.2-0.3 eV for the entire range of one- and two-electron excitations. (C) 2000 Elsevier Science B.V. All rights reserved.