Ion Pairing in Liquid Water: An <i>Ab Initio</i> Study

Potentials of mean force are widely used to study solvation characteristics as a function of a particular reaction coordinate of interest. In particular, ion-pair association in water is of great importance to many scientific and industrial applications, but is not widely understood. Here we present a detailed study of NaCl solvation properties in the dilute limit. We examine in detail the sources of discrepancies between simulation results, with the objective to separate approximation and statistical errors from physical interpretations. It is found that the choice of water model and simulation time step has a notable effect on the solvation state stability. Moreover, solution density also has a large effect on energetically favorable configurations. We show how the underlying description of liquid water has important consequences on the inter-ionic potential of mean force, and how the solute ions affect the solution's equilibrium properties. Our results shed light into how ions in water affect its structure and correlate with low density and high density liquid fluctuations.