First principles study of d→d excitations in bulk NiO

Direct calculations are reported of the one-electron d(xy) -> d(x(2) - y(2)) and d( xy)! d(z(2)) and two-electron d(xz, yz) -> d(z(2), x(2) - y(2)) crystal-field excitations in NiO based on first principles calculations within the B3LYP hybrid scheme. It is shown that within this scheme, calculations at the limit of exact exchange lead to the most satisfactory overall agreement with the experimental spectra, and that for the localized basis set used in this study the stability limit of the excited states is between 30% and 40% exact exchange for the AF(2) and similar to 20% for the FM spin alignments. A mapping of the crystal-field Hamiltonian suggested by Kanamori onto the directly calculated ground and excited state total energies leads to values of the crystal-field, Delta(CF), and the Racah B parameters which are close to those obtained previously from spectroscopic and theoretical studies. There is also close agreement between the two-electron excitation energy obtained from the Kanamori Hamiltonian and that calculated directly. It is shown that magnetic coupling constants obtained from a mapping of an Ising spin Hamiltonian onto ground state energies do not provide a quantitative guide to differences in the energies of the AF2 and FM spin alignments of excited states.