Second-Generation ReaxFF Water Force Field: Improvements in the Description of Water Density and OH-Anion Diffusion

Hydronium (H₃O⁺) and hydroxide (OH^-) ions have anomalously large diffusion constants in aqueous solutions due to their combination of vehicular and Grotthuss hopping diffusion mechanisms. An improvement of the ReaxFF reactive water force field on the basis of our first-generation water force field (water-2010) is presented to describe the proton transfer (PT) mechanisms of H₃O⁺ and OH^- in water. Molecular dynamics simulation studies with the water-2017 force field support the Eigen-Zundel-Eigen mechanism for PT in acidic aqueous solution and reproduce the hypercoordinated solvation structure of the OH^- in a basic environment. In particular, it predicts the correct order of the diffusion constants of H₂O, H₃O⁺, and OH^- and their values are in agreement with the experimental data. Another interesting observation is that the diffusion constants of H₃O⁺ and OH^- are close to each other at high concentration due to the strong correlation between OH^- ions in basic aqueous solution. On the basis of our results, it is shown that ReaxFF provides a novel approach to study the complex acid-base chemical reactions in aqueous solution with any pH value.