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 d irect total energy differences. Comparisons are made with optical absorptio n and electron energy-loss spectra and with previous multi-reference CEPA a nd CASSCF/CASPT2 calculations for (NiO6)(10-) and (NiO5)(8-) clusters. It i s shown that, as a result of their local character, the energies of these e xcitations are determined essentially by the coordination number of the exc ited atom rather than the nature of the local environment. From comparisons with the experimental data it is concluded that, for periodic/extended sys tems, the contribution from electron correlation is approximately 0.2-0.3 e V for the entire range of one- and two-electron excitations. (C) 2000 Elsev ier Science B.V. All rights reserved.