Free Energy Profile of NaCl in Water: First-Principles Molecular Dynamics with SCAN and ωB97X-V Exchange-Correlation Functionals.

Properties of water and aqueous ionic solutions are of great scientific interest because they play a central role in the atmosphere, biological environments, and various industrial processes. Employing two advanced exchange-correlation (XC) approximations, ωB97X-V and SCAN, in first-principles molecular dynamics simulations, we calculate the potential of mean force of NaCl in water as a function of the ion separation distance. Compared to the commonly used GGA-PBE functional, both of these XC functionals perform much better in simulating liquid water at room temperature for obtaining structural properties. The potential of mean force of NaCl in water exhibits two minima corresponding to two distinct types of ion pairing. ωB97X-V predicts that the contact ion pair is energetically more stable than the solvent-separated ion pair. The SCAN functional, however, predicts the opposite stability order, similarly to other XC functionals such as PBE. This is notable especially since classical molecular dynamics simulations with widely used force-field models predict greater stability for the contact ion pair. We also discuss how the electronic structures of water molecules and ions depend on the XC approximations. ωB97X-V and SCAN approximations noticeably improve the description of electron charge on Cl- ion in water while the charge on Na+ ion does not vary appreciably among the three XC functionals.