Reorganization energy associated with small polaron mobility in iron oxide

The reorganization energy is an important quantity controlling electron transfer rates. The internal contribution arising from the energy to reorganize donor/acceptor bonds can be evaluated by the "direct" and "4-point" methods. We examine how spatial separation leading to the noninteracting character of the donor and acceptor affects the reorganization energy. We show that the direct method captures contributions from interaction of the donor and acceptor while the 4-point method does not, and the two methods converge at large separation. Comparing reorganization energies determined by the two methods yields a measure of the degree of interaction between the initial and final states. The analysis is illustrated in the characterization of small polarons in iron oxides.