In continuum reaction field models of solvation, unconstrained quantum
mechanical calculation of the solute electronic structure inevitably
leads to penetration of some solute charge density outside the cavity
and into the solvent dielectric region. This produces a rarely recogni
zed or treated volume polarization that contributes in addition to the
commonly considered surface polarization. In this work a new practica
l implementation is described for quantitatively evaluating both volum
e and surface polarization contributions to the solute-solvent interac
tion with an irregularly shaped cavity surface. For illustration, nume
rical results are presented on several representative small neutral, c
ation, and anion solutes. The volume polarization contributions to ene
rgies and dipole moments are found to be somewhat smaller than those f
rom surface polarization, but not negligible. The results are also use
d to test several charge renormalization approaches that have been pre
viously proposed in the literature. Compared to the exact volume polar
ization correction, these can sometimes lead to energy corrections of
the wrong sign. A previously proposed method of simulating volume pola
rization through an additional surface polarization generally produces
corrections of the right sign and of about the right magnitude. (C) 1
998 American Institute of Physics.