Combining a Dissociative Water Model with a Hybrid QM/MM Approach—A Simulation Strategy for the Study of Proton Transfer Reactions in Solution

An implementation strategy for a dissociative water potential in hybrid QM/MM simulations is outlined. As the knowledge of the time-dependent topology is crucial for the assignment of solvent molecules to the QM or MM subregion, proton transfer events and the associated change of the molecular composition have to be monitored as the simulation is progressing. A simple and effective update criterion is proposed, which was found to be an efficient tool to identify sustained proton transfer reactions. The resulting topology data enable the application of the dissociative solvent model in QM/MM simulations and serve as a reference for the analysis of time-dependent properties such as the proton hopping rate and the diffusion coefficient. For the latter, an interpolation scheme is proposed linking subsequent proton transfer events into a single diffusive entity. Suitable settings of key parameters for the topology update and the interpolation scheme have been determined by analyzing MD trajectories of an excess proton in water and 1 M HCl. The resulting values for the proton hopping rate and the diffusion coefficient are well within the range estimated by EVB models and CPMD approaches. An investigation of the hydrolytic conversion of As(III) to [As(OH)2](+) serves as an exemplary application of the dissociative model in a QM/MM simulation study.