ORIENTATION-AVERAGED BENZENE BENZENE POTENTIAL OF MEAN FORCE IN AQUEOUS-SOLUTION

The potential of mean force as a function of the distance between two benzene molecules in aqueous solution has been calculated. Statistical mechanical averages were performed over the translational and rotational degrees of freedom of the solvent molecules as well as over the rotational degrees of freedom of the benzene molecules. The minimum of the potential of mean force was found to be -2.1 kJ mol-1 at 5.0-angstrom separation, at which the benzene molecules are preferentially perpendicularly oriented with respect to each other. At smaller separations the benzene molecules are forced to be more parallel, but the probability of these separations is small. Finally, the convergence of the average over the benzene orientations is considerably slower than that of the average over the solvent molecules, and this discrepancy will become more severe for more nonspherical or less symmetric solutes.