Application of trajectory surface hopping to the study of intramolecular electron transfer in polyatomic organic systems

An implementation of the trajectory surface hopping model is described that can be used to investigate hole or electron transfer in medium-sized organic molecules (up to about 50 atoms). The procedure employs direct dynamics with semiempirical (AM1) estimation of potential energy and its derivatives. The calculations involve configuration interaction between the lowest energy configurations and use the Landau-Zener model to estimate probabilities of hole or electron transfer at each site of avoided crossing between the diabatic configurations. In the present paper, the principles of the model are described and some qualitative insights based on single trajectories are discussed. A method for implementing a somewhat more rigorous version, using ensembles of trajectories, is also described. Application of the model to hole transfer in bridged policyclic radical cations reveals that the common assumption of a single, well-defined frequency for the encounter with the avoided-crossing region is sometimes justified but sometimes not, Support is also found for the concept of interference between hole-transfer paths within policyclic molecules-an idea that had been based on Koopmans' theorem analyses in earlier work.