A mean-field theory of a localized excess electron in a polar fluid

Abstract

A mean-field theory of a localized, excess electron in a classical, nonpolar fluid, presented in an earlier paper [J. Zhu and R. I. Cukier, J. Chem. Phys. 99, 1288 (1993)], is extended to polar fluids. The mean-field potential, and the effective potential, characterizing the electron–solvent interaction, are both modified by the addition of a long-ranged, attractive term arising from the charge–dipole interaction between the electron density and the solvent dipoles. The attractive part of this effective interaction is similar to that of an anion–dipole interaction, which makes possible the closure of the Ornstein–Zernike equations, characterizing the solvent–solvent and electron–solvent structure, by a suitably modified form of the mean spherical approximation, familiar from ion–dipole theories. The theory is compared with simulations of an excess electron in water, carried out with a new electron–solvent pseudopotential, designed to mimic the potentials of the mean-field theory. Agreement between the theory and simulation is good. Both theory and simulation predict that the solvation structure around the electron is weak. We find that the repulsive part of the force is dominant in the electron’s localization, and the long-ranged force serves only to contract the electron. We explore the contrast between the electron–solvation structure in water and a polar liquid with a more ideal dipole, to confirm that the deficiencies of the mean-field theory, constructed on the basis of an ideal-dipole fluid, are associated with the nonideality of the water dipole.