An empirical valence bond model for proton transfer in water

A new empirical valence bond model for proton transfer in bulk water that includes electron correlation effects is presented. The parameters of the model are based on ab initio calculations, in which electron correlation is treated at the MP2 level. Within this model, the properties of the gas-phase H₅O⁺₂ complex are in good agreement with recent ab initio path integral studies [M. E. Tuckerman, D. Marx, M. L. Klein, and M. Parrinello, Science 101, 4878 (1994)] and ab initio molecular dynamics studies [D. Wei and D. R. Salahub, J. Chem. Phys. 106, 6086 (1997)]. Simulations of the solvated H₅O⁺₂ complex suggest that at room temperature, the quantum nature of the transferring proton does not affect the essential mechanism of proton transfer and only slightly affects the free energy profile of the asymmetric stretch within the strong hydrogen bond. The predictions of the model are consistent with ab initio molecular dynamics simulations of solvated hydronium using gradient-corrected density functional theory [M. E. Tuckerman, D. Laasonen, M. Sprik, and M. Parrinello, J. Chem. Phys. 103, 150 (1995)].