Solvation free energies of polar molecular solutes: Application of the two-sphere Born radius in continuum models of solvation

A two-sphere description of the effective Born radius for spherical ions was found in previous work to yield accurate free energies for spherical ions. This effective Born radius (R-eff) was identified as the mean of the ionic radius (R-ion) and the distance to the first peak of the ion-oxygen/hydrogen radial charge or number density distribution function (R-gmax); i.e., R-eff=(R-ion+R-gmax)/2. To see whether this prescription also applies to the solvation of nonspherical polar molecules, it was used in finite-difference Poisson methods as well as in Kirkwood and generalized Born models to compute solvation free energies of model diatomic molecules of varying interatomic bond distances. Hydration free energies for the same model systems were also derived from free energy simulations in the presence of explicit water molecules. The good agreement between explicit solvent results and continuum solvent results with the two-sphere Born radius indicates that the latter description provides the required solute-solvent boundary in continuum solvent models. In contrast, using R-gmax alone to define the dielectric boundary in the three continuum solvent models yielded solvation free energies that deviated significantly from the respective simulation values. (C) 2001 American Institute of Physics.