Reaction field cavity optimization: A born-again Born model for ionic hydration

We present a new quantum reaction field model, the electronic reaction field (ERF), which does not require the empirical specification of cavity shape and size. We demonstrate our approach on one of the simplest reaction field theories, the Born model for aqueous atomic ion solvation. The ERF model enforces complete solute-solvent separability so that the solute wave function is optimized under the boundary condition that it is fully contained within the cavity. This model imposes the exact condition necessary for treating the solute quantum mechanically and the other as a macroscopic classical dielectric, and allows the testing of the primary assumption in the dielectric continuum approximation that the molecular character of solvent is unimportant. We show that the Born theory for monovalent and divalent cations is consistent with ERF model enforcing full wave function containment, while strict solute-solvent separability gives a poor description for anions of any valency. (C) 1999 American Institute of Physics. [S0021-9606(99)51245-2].