On the residence time for water in a solute hydration shell: application to aqueous halide solutions.

We investigate several different methods to determine the water residence time next to a solute from molecular dynamics simulations. The popular computational prescription due to Impey et al. ( J. Phys. Chem. 1983, 87, 5071-5083) is shown to be extremely sensitive to the t* tolerance time value (designed to account for barrier recrossing effects), and we evidence through a kinetic analysis that the conventionally employed t* = 2 ps value can yield seriously overestimated residence times for low barrier exchanges. We suggest an alternate, robust determination based on the stable states picture (SSP) of chemical reactions ( J. Chem. Phys. 1980, 73, 2700-2714) where recrossing is naturally discarded. This is illustrated by calculation of the water residence time next to a water molecule and next to a chloride ion, using both nonpolarizable and polarizable force-fields. The SSP results are in good agreement with the residence times estimated by a separate kinetic analysis, and differ noticeably from those calculated in the conventional fashion mentioned above.