Optimization of classical nonpolarizable force fields for OH⁻ and H₃O⁺

We optimize force fields for H₃O⁺ and OH⁻ that reproduce the experimental solvation free energies and the activities of H₃O⁺ Cl⁻ and Na⁺ OH⁻ solutions up to concentrations of 1.5 mol/l. The force fields are optimized with respect to the partial charge on the hydrogen atoms and the Lennard-Jones parameters of the oxygen atoms. Remarkably, the partial charge on the hydrogen atom of the optimized H₃O⁺ force field is 0.8 ± 0.1|e|—significantly higher than the value typically used for nonpolarizable water models and H₃O⁺ force fields. In contrast, the optimal partial charge on the hydrogen atom of OH⁻ turns out to be zero. Standard combination rules can be used for H₃O⁺ Cl⁻ solutions, while for Na⁺ OH⁻ solutions, we need to significantly increase the effective anion-cation Lennard-Jones radius. While highlighting the importance of intramolecular electrostatics, our results show that it is possible to generate thermodynamically consistent force fields without using atomic polarizability.