Abstract
The electronic coupling matrix element H(AB) is an essential ingredient of most electron-transfer theories. H(AB) depends on the overlap between donor and acceptor wave functions and is affected by the involved states' spin. We classify the spin-state effects into three categories: orbital occupation, spin-dependent electron density, and density delocalization. The orbital occupancy reflects the diverse chemical nature and reactivity of the spin states of interest. The effect of spin-dependent density is related to a more compact electron density cloud at lower spin states due to decreased exchange interactions between electrons. Density delocalization is strongly connected with the covalency concept that increases the spatial extent of the diabatic state's electron density in specific directions. We illustrate these effects with high-level ab initio calculations on model direct donor-acceptor systems relevant to metal oxide materials and biological electron transfer. Obtained results can be used to benchmark existing methods for H(AB) calculations in complicated cases such as spin-crossover materials or antiferromagnetically coupled systems.