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A recently proposed impuritylike many-body theory for the electronic structure of transition-metal compounds is extended to core photoemission. The theory is worked out in detail for the nickel dihalides, and we compare the results with experimental 2p spectra. We show that these spectra can be used for a quantitative determination of the parameters in the theory. We furthermore show that the easy-to-handle cluster model is a fair approximation to the full theory as far as core x-ray photoemission is concerned. We find that the Coulomb interaction energies are the largest energies in the system, and we show that the order of magnitude of these can be estimated from an ionic screening model. Furthermore we show that the charge-transfer energies strongly vary along the series. A comparison with estimates for the parameters derived from other experiments show systematic discrepancies. We argue that these are due to the neglect of higher-order interactions in the model Hamiltonian used.
Zaanen et al. (Sun,) studied this question.