Ionic hydration at ambient and higher pressures: Computed chemical potentials from simulations and finite-size effects

For the chemical potential of a hydrated mono-atomic ion, finite- size effects on simulation results obtained using molecular potential truncation are investigated. Free energy perturbation (FEP) calculations were carried out by scaling in two processes the Lennard-Jones (LJ) ion-water parameters and the ion-charge (q) for Br-, K+ and Ca2+ interacting with TIP4P water. Corrections which scale with q2 enable us to reduce finite-size effects. However, at ambient conditions, discrepancies which depend on q are shown by the corrected values when comparison is made with the experimental data of the Marcus compilation. Similar behavior was observed by extrapolating the original FEP results to an infinitely large system. Hence, these errors were assumed to depend on water density and corrected at high pressures. Consistency, within statistical uncertainties, is shown when comparing with results derived from computed volumetric quantities. Results are also compared with those derived from experimental values of excess volumes at ambient conditions.