  On the basis of a gaussian quasi-chemical model of hydration, a model of non
van der Waals character, we explore the role of attractive methane-water
interactions in the hydration of methane and in the potential of mean force
between two methane molecules in water. We find that the hydration of methane
is dominated by packing and a mean-field energetic contribution. Contributions
beyond the mean-field term are unimportant in the hydration phenomena for a
hydrophobic solute such as methane. Attractive solute-water interactions make a
net repulsive contribution to these pair potentials of mean force. With no
conditioning, the observed distributions of binding energies are super-gaussian
and can be effectively modeled by a Gumbel (extreme value) distribution. This
further supports the view that the characteristic form of the unconditioned
distribution in the high-e tail is due to energetic interactions with a small
number of molecules. Generalized extreme value distributions also effectively
model the results with minimal conditioning, but in those cases the
distributions are sufficiently narrow that details of their shape aren't
significant.
