Monte Carlo simulation of differences in free energies of hydration

Perturbation theory has been applied to calculate the relative free energies of hydration of methanol and ethane in dilute soluton. It is demonstrated that only two or three Monte Carlo simulations using double-wide sampling are necessary to obtain results with high precision. The small statistical uncertainty in the computed change in free energy of hydration and the good accord with experimental thermodynamic data are most encouraging for application of the procedure to a wide range of problems. Structural effects accompanying the mutation of methanol to ethane in water are also discussed; hydrogen bonding to the solute is essentialy eliminated by only a 25% reduction in the atomic charges of methanol.