Rate-limiting step of proton transport in liquid water
In plain words
Extra hydrogen ions move through water several times faster than other ions by hopping between water molecules. Which molecular step sets the speed of this hopping is not agreed.
Precise statement
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 $9.3e-5\,\mathrm{cm}^2/\mathrm{s}$) and its H/D isotope effect and identifies the step from the trajectories.
What would settle it
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.