Accurate modeling of liquid water and proton transfer in water

Water is of the utmost importance for life and proton transfer via hydronium and hydroxide ions in water is also ubiquitous. A genuinely predictive ab initio model of water is the key to understanding the proton transfer effect in water. However, accurate prediction of water requires to climb up the Jacob's ladder within density functional theory to include the treatment of van der Waals interactions and the mitigation of self-interaction error. We demonstrate that a fully ab initio approach, relying on the strongly constrained and appropriately normed (SCAN) density functional, provides such a description of water. SCAN accurately describes the balance among covalent bonds, hydrogen bonds, and van der Waals interactions that dictates the structure and dynamics of liquid water. At the similar level of theory, we then show that structural diffusion of hydronium preserves the previously recognized concerted behavior. However, by contrast, proton transfer via hydroxide is dominated by stepwise events, arising from a stabilized hyper-coordination solvation structure that discourages proton transfer. Specifically, the latter exhibits non-planar geometry, which agrees with neutron scattering results. Asymmetry in the temporal correlation of proton transfer enables hydronium to diffuse faster than hydroxide and may underlie observed isotope anomalies.