Characterizing the free energy landscape of water ionization has been a great challenge due to the limitations from expensive ab initio calculations and strong rare-event features. Lacking equilibrium sampling of the ionization pathway will cause ambiguities in the mechanistic study. Here, we obtain convergent free energy surfaces through nanosecond timescale metadynamics simulations with classical nuclei enhanced by atomic neural network potentials, which yields good reproduction of the equilibrium constant ($\mathrm{p}{K}_{\mathrm{w}}=14.14$) and ionization rate constant ($1.369\times {10}^{-3}{\mathrm{s}}^{-1}$). The character of transition state unveils the triple-proton transfer occurs through a concerted but asynchronous mechanism. Conditional ensemble average analyses establish the dual-presolvation mechanism, where a pair of hypercoordinated and undercoordinated waters bridged by one ${\mathrm{H}}_2\mathrm{O}$ cooperatively constitutes the initiation environment for autoionization, and contributes extremely to the local electric field fluctuation to promote water dissociation.

• Received 2 August 2022
• Revised 5 July 2023
• Accepted 21 August 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.158001