Calculation of x-ray-absorption spectra of strongly correlated systems

The L-2,L-3 edge x-ray-absorption spectra for transition-metal compounds ar e usually calculated retaining only those states in the ground-state wave f unction which correspond to a purely ionic configuration at the transition- metal site, as well as those arising from a single electron transfer from t he surrounding ligand sites. We critically compare such a truncated basis a pproach to a complete basis approach, which is exact within the assumed clu ster model, and discuss the limitations of the approximate method, specific ally in the case of compounds with the transition-metal ion in a formal hig h oxidation state. It is shown that while the truncated basis approach may be adequate for divalent transition-metal compounds, it can lead to erroneo us estimates for parameter strengths in the case of highly oxidized, and co nsequently, strongly covalent compounds.