Core-hole effect in the one-particle approximation revisited from density functional theory

The strength of the core-hole effect in simulations of electron energy-loss near edge structures or x-ray absorption near edge structures is investigated using ab initio calculations based on the density functional theory. Calculations were performed using the WIEN2K and FEFF codes at different edges for the following model compounds: rutile TiO2, MgO, and for some transition metals (Ti, Fe, Co, Cu, and Zn). We demonstrate that although a core hole is always present in any core-level spectroscopy experiment, its effect is not always observable. To observe or not a core-hole effect in any experimental situation can, however, be predicted from the examination of the material's ground-state electronic structure, especially of the states just above the Fermi level. Indeed, it is shown that the two important criteria governing the core-hole strength at a given edge depend first on the localization and on the character of the first empty states of the material under investigation, and second on the nature of the compensating charge necessary to maintain the crystal neutrality. These criteria are expected to give a good description of the core-hole effect in the one-particle approximation. However, nothing can be inferred as for the good/bad agreement between calculations and experiments for more complex cases such as excitations of delocalized semicore states or of atomiclike multiplet structures.