A Monte Carlo method for obtaining the interionic potential of mean force in ionic solution

A Monte Carlo method for obtaining the solvent-averaged interionic potential of mean force is described. The potential of mean force is obtained for two charged hard spheres immersed in a dipolar hard sphere solvent. The ions and solvent particles have the same hard sphere diameters and the ions bear single charges of opposite sign. The solvent particles are characterized by a reduced dipole moment μ*=1.0 which corresponds to a dielectric constant ε?7.8. The Monte Carlo result is compared with the ’’primitive model’’ and other approximations which have been suggested for the potential of mean force. These approximations are all found to be inadequate for small ionic separations. Some implications with respect to ’’ion pairing’’ are mentioned.