Electron–hole excitations in NiO: LSDA+U-based calculations vs. inelastic X-ray scattering and ellipsometry measurements

The performance of the LSDA+U functional—in particular, the quality of the ground-state—is tested via calculations of the electron–hole excitations of NiO, which are compared with (non-resonant) inelastic X-ray scattering (IXS) and ellipsometry measurements. The dynamical density-response calculations are performed within the random-phase approximation (RPA), defining an LSDA+U/RPA density-response method. A significant success of this method is the insight it provides into the main loss present in the IXS data above the NiO optical gap, namely, a peak lying at ∼7.5 eV. This excitation, which is shown to be collective in nature, and to be induced by eg–eg transitions, provides a direct link between the correlated eg-states and the IXS data. This finding illustrates the power of IXS, combined with correlated-band-structure theory (here, LSDA+U theory), for the investigation of the electronic structure of strongly correlated materials. At the same time, our results indicate that the LSDA+U/RPA response method does not represent a complete theory.