  We study ion pair dissociation in water at ambient conditions using a
combination of classical and ab initio approaches. The goal of this study is to
disentangle the sources of discrepancy observed in computed potentials of mean
force. In particular we aim to understand why some models favor the stability
of solvent-separated ion pairs versus contact ion pairs. We found that some
observed differences can be explained by non-converged simulation parameters.
However, we also unveil that for some models, small changes in the solution
density can have significant effects on modifying the equilibrium balance
between the two configurations. We conclude that the thermodynamic stability of
contact and solvent-separated ion pairs is very sensitive to the dielectric
properties of the underlying simulation model. In general, classical models are
very robust on providing a similar estimation of the contact ion pair
stability, while this is much more variable in density functional theory-based
models. The barrier to transition from solvent-separated to contact ion pair is
fundamentally dependent on the balance between electrostatic potential energy
and entropy. This reflects the importance of water intra- and inter-molecular
polarizability in obtaining an accurate description of the screened ion-ion
interactions.
