Piepho–Krausz–Schatz model

The Piepho-Krausz-Schatz model or PKS is applied to mixed valence systems with vibronic coupling, especially to transition coordination compounds.

This model, first put forward by Susan B. Piepho, Elmars R. Krausz, and P. N. Schatz in 1977, takes into account the more symmetrical, "breathing" around the electroactive centers. The main contribution about the Hush model, above, is that with the PKS model explicit eigenfunctions and eigenvalues are obtained (it is a model Hamiltonian). Thus, in addition to the position and intensity of the interval band, it is possible to calculate its structure and shape, as well as other properties, such as the EPR spectrum, magnetic properties, or the degree of electronic delocalization.

The approach of the ligands becomes more unfavorable energetically if there is an "extra" electron in the center. Thus, the vibronic coupling of PKS competes with electron transfer, and has a localizing effect.