Chloride ion pairs in water

The study of ions in water by statistical mechanical methods has made a significant contribution to the authors understanding of solution chemistry and biological processes in saline solutions. Integral equation methods have been used recently by Pettitt and Rossky to study solvent-averaged forces and the effective interactions or the potentials of mean force (PMF) for the alkali halides in water at infinite dilution. In this communication, they report a quantitative study of the Cl/sup -/-Cl/sup -/ PMF in water with use of an umbrella sampling method and the same Hamiltonian as that used in the integral equation study. The system studied here consists of two chloride ions and 295 water molecules in a rectangular box with periodic boundary conditions and lengths of 25.4, 18.6, and 18.6 A in the x,y,z directions, respectively.