PROTON-TRANSFER IN SOLUTION - MOLECULAR-DYNAMICS WITH QUANTUM TRANSITIONS

We apply ''molecular dynamics with quantum transitions'' (MDQT), a sur face-hopping method previously used only for electronic transitions, t o proton transfer in solution, where the quantum particle is an atom. We use full classical mechanical molecular dynamics for the heavy atom degrees of freedom, including the solvent molecules, and treat the hy drogen motion quantum mechanically. We identify new obstacles that ari se in this application of MDQT and present methods for overcoming them . We implement these new methods to demonstrate that application of MD QT to proton transfer in solution is computationally feasible and appe ars capable of accurately incorporating quantum mechanical phenomena s uch as tunneling and isotope effects. As an initial application of the method, we employ a model used previously by Azzouz and Borgis to rep resent the proton transfer reaction AH-B reversible arrow A(-)-H+B in liquid methyl chloride, where the AH-B complex corresponds to a typica l phenol-amine complex. We have chosen this model, in part, because it exhibits both adiabatic and diabatic behavior, thereby offering a str ingent test of the theory. MDQT proves capable of treating both limits , as well as the intermediate regime. Up to four quantum states were i ncluded in this simulation, and the method can easily be extended to i nclude additional excited states, so it can be applied to a wide range of processes, such as photoassisted tunneling. In addition, this meth od is not perturbative, so trajectories can be continued after the bar rier is crossed to follow the subsequent dynamics.