  K+(aq) ion is an integral component of many cellular processes, amongst which
the most important, perhaps, is its role in transmitting electrical impulses
along the nerve. Understanding its hydration structure and thermodynamics is
crucial in dissecting its role in such processes. Her we address these
questions using both the statistical mechanical quasi-chemical theory of
solutions and ab initio molecular dynamics simulations. Simulations predict an
interesting hydration structure for K+(aq): the population of about six (6)
water molecules within the initial minimum of the observed g_{KO}(r) at
infinite dilution involves four (4) inner-most molecules that the
quasi-chemical theory suggests should be taken as the theoretical inner shell.
The contribution of the 5th and 6th closest water molecules is observable as a
distinct shoulder on the principal maximum of the g_{KO}(r). The quasi-chemical
estimate of solvation free energy for the neutral pair KOH is also in good
agreement with experiments.
