Free Energy of the Hydrophobic Interaction from Molecular Dynamics Simulations: The Effects of Solute and Solvent Polarizability

Molecular dynamics simulations are used to calculate the free energy of methane association in water, using the polarizable fluctuating charge model that treats the charges on atomic sites as dynamical variables. Compared with previous studies using nonpolarizable potentials, the inclusion of polarizability leads only to small differences in the methane pair potential of mean force. This is in contradistinction to two previous studies using other polarizable models, which do not agree with the nonpolarizable results or with each other. The potential of mean force is calculated at three different temperatures (283, 298, and 313 K) from which the temperature dependence and also the entropic part of the free energy is examined. It is found that the tendency for methane molecules to aggregate increases with increasing temperature and that aggregation is stablized by entropy.