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Which molecular step sets the speed of this hopping is not agreed.","posed_since":"","precise":"For an excess proton in liquid water at $300\\,\\mathrm{K}$, identify the rate-limiting step of structural (Grotthuss) diffusion, the relay of the proton from molecule to molecule: hydrogen-bond breaking or forming in the second solvation shell, or motion along the proton-transfer coordinate. An answer is a simulation with path-integral nuclei on a near-quantum-chemical potential that reproduces the measured proton diffusion coefficient (about $9.3 \\times 10^{-5}\\,\\mathrm{cm}^2/\\mathrm{s}$) and its H/D isotope effect and identifies the step from the trajectories.","problem_ref":"86f9acae4a5d4803df0e1fd08f217046ce969911fb81c49342f61c1524a312eb","references":"","settled_by":"Path-integral molecular dynamics on a coupled-cluster-quality potential reproducing diffusion and isotope effect, with a committor or rate analysis of the transfer events.","status_note":"","title":"Rate-limiting step of proton transport in liquid 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