The electronic structure of NiO for Ni 3s-hole states including full orbital relaxation and localization

The electronic structure of NiO, with emphasis on the Ni 3s-hole ionic states, is studied using non-orthogonal configuration interaction, NOCI, wavefunctions for an NiO6 model of the crystal. Orbital sets are relaxed, or optimized, separately for each configuration used in the NOCI and orbital symmetry breaking, or localization, is allowed. This localization is important for configurations that involve large amounts of charge transfer from O(2p) to the Ni(3d) shell. The NOCI method insures an unbiased treatment of the relative energies of configurations that involve different degrees of charge transfer from O(2p) to Ni(3d). The use of fully relaxed orbitals is shown to be necessary to obtain accurate energies and intensities for core level ionic states observed with X-ray photoelectron spectroscopy, XPS. The NOCI energies and intensities for the lowest and first excited, high spin coupled, 3s-hole states are in good agreement with XPS spectra. Both high spin 3s-hole states are found to have significant, but partial, charge transfer character.