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Ab initio multiplet plus cumulant approach for correlation effects in x-ray photoelectron spectroscopy

The treatment of electronic correlations in open-shell systems is among the most challenging problems of condensed matter theory. Current approximations are only partly successful. Ligand field multiplet theory (LFMT) has been widely successful in describing intra-atomic correlation effects in x-ray spectra, but typically ignores itinerant states. The cumulant expansion for the one electron Greens function has been successful in describing shake-up effects but ignores atomic multiplets. More complete methods, such as dynamic mean-field theory can be computationally problematic. Here we show that separating the dynamic Coulomb interactions into local and longer-range parts with ab initio parameters yields a combined multiplet plus cumulant approach that accounts for both local atomic multiplets and satellite excitations. The approach is illustrated in transition metal oxides and explains the multiplet peaks, charge-transfer satellites and distributed background features observed in XPS experiment.