CASSCF study of the relation between the Fe charge and the Mِssbauer isomer shift

Multiconfigurational wave functions are calculated for a series of Fe complexes. We find a linear correlation between the experimental 57Fe Mِssbauer isomer shift and the calculated electron density at the Fe nucleus. However, the analysis of the wave function in valence bond terms shows that there is no straightforward relation between the density at the nucleus and the Fe charge. The analysis of the CASSCF wave function expressed in localized orbitals shows that the isomer shift is very sensitive to the weight of charge transfer configurations and hence to the covalency, rather than to the absolute charge.