Bringing Reactivity to the Aggregation-Volume-Bias Monte Carlo Based Simulation Framework: Water Nucleation Induced by a Reactive Proton.

Abstract

The development of the aggregation-volume-bias Monte Carlo based simulation technique has led to recent success in studying rare nucleation events, but thus far, this simulation method has been limited to nonreactive systems. This work presents the first application of this technique to study a reactive system of relevance to atmospheric chemistry, i.e., formation of water droplets in the presence of a reactive proton, by combining this approach with a multistate empirical valence bond (MSEVB) description of the excess proton (or the hydronium). It was shown that the ability for the hydronium to share its charge with adjacent water molecules changes dramatically with the cluster size, especially when clusters are small and the distribution of the charge is affected by the presence of an interface, emphasizing the need to use this more sophisticated MSEVB model for such a reactive system. In addition, the simulation results obtained from this system are compared to those with nonreactive hard-sphere ions of different sizes. Overall, the presence of a hydronium or ions appeared to dramatically change the free energy landscape of nucleation compared to the pure water system, leading to the formation of a stable precritical cluster. Although the free energy change due to the addition of the first few water molecules was shown to be very sensitive to the ionic details, the later portion of the free energy profile was found to be nearly independent of the nature of the ion.