Classical Models of Hydroxide for Proton Hopping Simulations

Hydronium (H₃O⁺) and hydroxide (OH^-) ions perform structural diffusion in water via sequential proton transfers ("Grotthuss hopping"). This phenomenon can be accounted for by interspersing stochastic proton transfer events in classical molecular dynamics simulations. The implementation of OH^--mediated proton hopping is particularly challenging because classical force fields are known to produce overcoordinated solvation structures around the OH^- ion. Here, we first explore the ability of two-particle point-charge models to reproduce both the solvation free energy and coordination number in TIP3P water. We find that this is possible only with unphysical changes in the nonbonded parameters which create problems in proton hopping simulations. We then construct a classical OH^- model with the charge of oxygen distributed among three auxiliary particles. This model favors a lower coordination number by accepting three hydrogen bonds and weakly donating one. The model was implemented in the MOBHY module of the CHARMM program and was fit to reproduce the experimental aqueous diffusion coefficient of OH^-. This parameterization gave reasonable electrophoretic mobilities and the expected accelerated transport under nanoconfinement.