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

Abstract

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.

Quantifying the hydration structure of sodium and potassium ions: taking additional steps on Jacob’s Ladder

“…The effects associated with quantum nuclear effects on the oxygen-oxygen RDF for water have been shown to be quite small for high quality interaction potentials. [51][52][53] The slightly over-structured behaviour of SCAN observed here is consistent with the work of Yao and Kanai 38,54 who also simulated at 300 K with CP2K 55 and CPMD 56 demonstrating good agreement between the two basis set approaches, namely gaussian functions and plane-waves. Simulation of 20 ps utilizing SCAN water in a slab configuration at 300 K using the protocol discussed herein is consistent with the density of 1.05 g/cm 3 calculated by Chen et al.…”

“…As previously stated, earlier research has argued that SCAN can accurately reproduce bulk water structure 42,43 but this appears to require unphysically elevating the temperature by + 30 K. Similarly, the Na-Cl potential of mean force (PMF) in water has been computed with SCAN. 38 Different functionals may describe anion-water and cation-water interactions with signif-icantly different accuracy. 24 GGAs appear to already adequately reproduce the structure of halide ions such as iodide.…”

“…SCAN doesn't appear to reproduce the NaCl dimer binding energy in vacuum as accurately as the wB97X-V functional 38. However, relatively small differences are observed when SCAN is compared to other traditional DFT functionals and there are no direct comparisons of the outcomes of the PMF with experimental results to demonstrate any improvement.…”

Measuring surface charge: Why experimental characterization and molecular modeling should be coupled

“…The main drawback of this technique has thus far been its large computational cost, which strongly limits the accessible simulation time and system sizes. Although the accessible time scale in AIMD is insufficient to directly probe transport properties, AIMD could instead be used to explore free energy profiles [193,194]. Alternatively, the limitation of accessible time scales can be mitigated using machine learning, e.g., to predict infrared spectra [195].…”

Looking Beyond Energy Efficiency: An Applied Review of Water Desalination Technologies and an Introduction to Capillary-Driven Desalination

“…Dissolution of most salts in water is enthalpically negative (∆H < 0) and entropically positive (∆S > 0) and thus a spontaneous process (∆G < 0) [80][81][82][83][84].…”