Rational determination of charge distributions for free energy calculations

Point charges derived from RHF/6-31G* electrostatic potentials are attractive because they tend to exaggerate the polarity of solvated molecules, thereby compensating in an average fashion missing induction effects. In the context of free energy calculations, wherein the molecule is transferred from a polar environment to a nonpolar one, we propose a more rational approach based on a self-consistent reaction field computation at a higher level of theory, supplemented by an estimation of the corresponding distortion energy to account for the change of polarity of the surroundings. Application of this method to the test cases acetamide, acetic acid, methyl acetate and phenol, using multinanosecond molecular dynamics/"umbrella sampling" simulations, yields consistent hydration free energies in reasonably good agreement with experiment.