  The aqueous proton displays an anomalously large diffusion coefficient that
is up to 7 times that of similarly sized cations. There is general consensus
that the proton achieves its high diffusion through the Grotthuss mechanism,
whereby protons hop from one molecule to the next. A main assumption concerning
the extraction of the timescale of the Grotthuss mechanism from experimental
results has been that, on average, there is an equal probability for the proton
to hop to any of its neighboring water molecules. Herein, we present ab initio
simulations that show this assumption is not generally valid. Specifically, we
observe that there is an increased probability for the proton to revert back to
its previous location. These correlations indicate that the interpretation of
the experimental results need to be re-examined and suggest that the timescale
of the Grotthuss mechanism is significantly shorter than was previously
thought.
