Optical transitions and the Mössbauer parameters isomer shift and quadrupole splitting have been compared with corresponding values computed from molecular orbital cluster calculations for ferric and ferrous ions in several minerals. Clusters comprised only Fe and the neighboring oxygen ions. Two molecular orbital methods–the iterative extended Hückel theory (ieht) and the multiple scattering Xα (MS-Xα) method–were used.
Values derived from the ieht method follow the trends that are predetermined by measured values, with the exception of the sign of the quadrupole interaction in gillespite. This failure is attributed to the inadequacy of the simple ieht method to represent the electronic structure of gillespite. The situation considerably improves after taking into account configuration interaction among the five low-lying quintet states of the ferrous high-spin cluster.
The more sophisticated Ms-Xα method has only been successful in describing optical data and isomer shifts, whereas calculated quadrupole splittings are unsatisfactory. Variations in the parameters did not improve this situation; therefore we suspect that the observed limitation of our MS-Xα results is due to the small cluster size used here.