The solvation of Na+ in water: First-principles simulations

First-principles molecular dynamics simulations have been performed on the solvation of ${\mathrm{Na}}^{+}$ in water. Consistent with the available experimental data, we find that the first solvation shell of ${\mathrm{Na}}^{+}$ contains on average 5.2 water molecules. A significant number of water exchanges between the first and second solvation shells are observed. However, the simulations are not long enough to reliably measure the rate of water exchange. Contrary to several previous studies, we do not find any effect of ${\mathrm{Na}}^{+}$ on the orientation of water molecules outside of the first solvation shell. Furthermore, the complete set of structural properties determined by first-principles molecular dynamics is not predicted by any of the known classical simulations.