Ab initio calculation of the potential of mean force for dissociation of aqueous Ca-Cl.

The potential of mean force for the dissociation of a Ca-Cl ion pair in water is calculated from ab initio molecular dynamics simulations. The constraint-force method is employed to enhance sampling over the entire range of the reaction coordinate (Ca-Cl distance) from 2.2 to 6.5 Å. Particular attention is paid to equilibration of the system as it is found that the potential of mean force is highly sensitive to the hydration number of the Ca(2+) ion. The structure and polarization of hydration waters are examined in detail at three ion-ion separations of interest: the contact-ion position, the solvent-separated-ion position, and the transition state between them. The ab initio results are compared to the classical ones obtained using the CHARMM force field and the parameters of Dang and Smith. There are substantial differences between the polarization of hydration waters of Ca(2+) and Cl(-) ions at all distances, which indicates that an accurate description of Ca-Cl dissociation with nonpolarizable force fields may not be feasible. The ab initio results presented here for the Ca-Cl ion pair complements our earlier results for Na-Cl, and together they provide useful benchmarks for polarizable force fields under construction.