Electron correlation effects in reduced or oxidized mixed-valency molecules

The vibronic model ordinarily written for mixed-valence molecules predicts an intervalence transfer (IVT) frequency which, in the limit of delocalized states, is unchanged upon reduction by one further electron. This is in sharp disagreement with experiment, but can be remedied, qualitatively, by adding one-site Coulomb repulsion which shifts the analog of the IVT into the near ultraviolet, and an exchange term which produces the correct ordering of the energy levels, thus making a transition analogous to IVT spin-forbidden.