Classical Models of Hydroxide for Proton Hopping Simulations

Hydronium (H<sub>3</sub>O<sup>+</sup>) and hydroxide (OH<sup>–</sup>) ions\nperform structural diffusion in water via sequential proton\ntransfers (“Grotthuss hopping”). This phenomenon can\nbe accounted for by interspersing stochastic proton transfer events\nin classical molecular dynamics simulations. The implementation of\nOH<sup>–</sup>-mediated proton hopping is particularly challenging\nbecause classical force fields are known to produce overcoordinated\nsolvation structures around the OH<sup>–</sup> ion. Here, we\nfirst explore the ability of two-particle point-charge models to reproduce\nboth the solvation free energy and coordination number in TIP3P water.\nWe find that this is possible only with unphysical changes in the\nnonbonded parameters which create problems in proton hopping simulations.\nWe then construct a classical OH<sup>–</sup> model with the\ncharge of oxygen distributed among three auxiliary particles. This\nmodel favors a lower coordination number by accepting three hydrogen\nbonds and weakly donating one. The model was implemented in the MOBHY\nmodule of the CHARMM program and was fit to reproduce the experimental\naqueous diffusion coefficient of OH<sup>–</sup>. This parameterization\ngave reasonable electrophoretic mobilities and the expected accelerated\ntransport under nanoconfinement.