TRANSFERABILITY OF ION MODELS

Parameters were derived empirically for K+ and Na+ that qualitatively reproduce the absolute free energies of solvation and structural characteristics of these ions in methanol. The procedure used free energy perturbation simulations to obtain the absolute free energies of solvation and molecular dynamics trajectories to examine structural aspects. We find that within the effective two-body model we obtain a reasonable set of compromise parameters. Our results also suggest that we have done as well as we can with an effective two-body potential. Further improvement must come from the use of methods that handle polarization, long-range electrostatic effects, and large simulation sample sizes. The results for these parameters are compared to other parameters (J. Phys. Chem. 1990, 94, 8021) that were derived using a similar strategy (i.e., fitting the free energy of solvation and structural data), but used an alternative simulation protocol. We find that ion parametrization is very dependent on computational strategy, which suggests that ion parameter sets may not be generally transferable.