Cluster embedding of ionic systems: Point charges and extended ions.

The embedding of cluster models of oxides with point charges and with extensions of the embedding which take into account the spatial extent of the cations is examined with an emphasis on the consequences of this embedding for the relative ionization and excitation energies that are measured in core-level spectroscopies. It is found that the dependence of the electronic structure of the oxides and the relative energies of different levels depend only weakly on the embedding and that relatively simple embeddings may be sufficient to provide an adequate model for determining core-level spectra. This is different from absolute values of the ionizations which, as expected, depend strongly on the details of the extended crystal; however, relative values of binding energies, as measured in photoemission, are of greater interest than the absolute values.