POTENTIAL OF MEAN FORCE FOR A SODIUM DIMETHYL-PHOSPHATE ION-PAIR IN AQUEOUS-SOLUTION - A FURTHER TEST OF THE EXTENDED RISM THEORY

We have applied the extended reference interaction site model (RISM) theory with the hypernetted chain (HNC) closure to evaluate the solvent structure and potential of mean force (PMF) for the sodium dimethyl phosphate (NaDMP) ion pair in ''simple point charge'' (SPC) water. Comparison is made between the free energies obtained from the RISM-HNC theory, from the so-called Gaussian bath theory using RISM solvent structure, and from computer simulation. Both the extended RISM theory and Gaussian bath theory predict the locations of PMF ''trema in reasonable accord with those calculated by molecular dynamics simulation, although the PMFs obtained from the extended RISM theory show smaller amplitudes. Overall, the Gaussian bath theory yields the PMF curves in better accord with simulation. However, both estimators of the free energy fail to predict the size of the relatively unfavorable free energy associated with multiple atomic contacts between the Na+ and the molecular anion. We attribute this to the shortcomings of the RISM-HNC ion-solvent correlation functions in such situations.