The near-edge structure in energy-loss spectroscopy: many-electron and magnetic effects in transition metal nitrides and carbides

We investigate the ability of the local density approximation (LDA) in dens ity functional theory to predict the near-edge structure in electron energy -loss spectroscopy in the dipole approximation. We include screening of the core hole within the LDA using Slater's transition state theory. We find t hat anion K-edge threshold energies are systematically overestimated by 4.2 2 +/- 0.44 eV in twelve transition metal carbides and nitrides in the rock- salt (B1) structure. When we apply this 'universal' many-electron correctio n to energy-loss spectra calculated within the transition state approximati on to LDA, we find quantitative agreement with experiment to within one or two eV for TiC, TiN and VN. We compare our calculations to a simpler approa ch using a projected Mulliken density which honours the dipole selection ru le, in place of the dipole matrix element itself. We find remarkably close agreement between these two approaches. Finally, we show an anomaly in the near-edge structure in CrN to be due to magnetic structure. In particular, we find that the N K edge in fact probes the magnetic moments and alignment s of ther sublattice.