A Reactive Molecular Dynamics Algorithm for Proton Transport in Aqueous Systems

Abstract

We present a model that incorporates the structural diffusion of a proton into a classical molecular dynamics simulation using a reactive molecular dynamics (RMD) algorithm. The transition state for proton transfer obtained from ab initio calculations is mapped onto a set of geometric and energetic triggers to describe the structural diffusion of the proton in the simulation. Numerical values of these triggers are parametrized to satisfy the experimental values of rate constant and activation energy in order to capture the molecular and macroscopic features of structural diffusion. The algorithm partitions the structural diffusion of a proton into three steps: (i) satisfaction of the triggers, (ii) instantaneous reaction, and (iii) local equilibration. The final step ensures that the ending point of the reaction provides the correct structure and heat of reaction. Hence, the reactivity is incorporated by the algorithm rather than through the development of a reactive potential. We have applied this scheme to study proton transport in bulk water and solutions of HCl. Total charge diffusion along with the structural and vehicular decomposition is studied as a function of temperature (280−320 K). The two components are found to be uncorrelated and the structural diffusion contributes 60−70% of the total charge diffusion in bulk water. The method is applied to HCl solutions (0.22−0.83 M) to study the effect of concentration on proton transport. The reduction in the total diffusivity of the charge with an increase in concentration is due to the reduction in structural diffusion.