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].