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

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