Mixed quantum/classical dynamics of single proton, multiple proton, and proton-coupled electron transfer reactions in the condensed phase

This chapter centers on the mixed quantum/classical molecular dynamics with quantum transitions (MDQT) method for simulating proton transfer reactions in the condensed phase. In this method the transferring hydrogen atom(s) are treated quantum mechanically, while the remaining nuclei are treated classically. The MDQT method incorporates nonadiabatic transitions among the proton vibrational states to allow the accurate description of branching processes (i.e. processes involving multiple pathways). This chapter presents a comparison of the MDQT method to fully quantum mechanical calculations for model systems representing single proton, double proton, and proton-coupled electron transfer reactions. The agreement between the MDQT and the fully quantum mechanical calculations provides validation for the application of MDQT to these biologically important processes. Also described is the multiconfigurational MDQT (MC-MDQT)? method, which combines a multiconfigurational self-consistent-field (MC-SCF) formulation of the adiabatic wavefunctions with the MDQT method for the simulation of multiple proton transfer reactions. This methodology has been utilized to investigate the real-time nonequilibrium quantum dynamics of proton transport along protonated chains of water molecules. The application of MDQT to model proton-coupled electron transfer reactions is also discussed. In this case the MDQT method incorporates nonadiabatic transitions among mixed proton/electron adiabatic quantum states. The methodology described is applicable to a wide range of chemically and biologically important processes.