On the ab-initio calculation ofd-d spectra in transition metal compounds: The importance of relaxed charge transfer states

Abstract

By calculations on CuCl, CuBr, and NiO clusters it is shown that a first-order configuration interaction (CI) calculation significantly improves the d-d and charge transfer (CT) spectra of ionic transition metal compounds. The first-order CI introduces delocalization (covalency) effects in the dn states, thus increasing the effective ligand-field splitting which is always underestimated at the Hartree–Fock (HF) level. It is demonstrated that this HF + first-order CI treatment is strongly related to a valence bond model. In this model the delocalization is introduced by explicit interactions with relaxed CT states. After account has been taken of the physically very different atomic correlation effects, a very good agreement with experimental d-d spectra is obtained, using only a small cluster. The effect of first-order CI on CT states is to account for hole localization and polarization effects which lead to reductions in the CT excitation energies in the order of 2–3 eV.