Importance of nuclear quantum effects on the hydration of chloride ion

The solvation structure of Cl^- ion in aqueous solution is determined by the H-bonding between the solvated ion and surrounding water molecules, which is in competition with the water-water H-bonds in the solvent. We performed ab initio path-integral molecular dynamics simulations based on the SCAN functional. We find that quantum nuclei tilt the above balance compared to conventional classical simulations. Nuclear quantum effects (NQE) weaken the ion-water H-bonding strengths as shown by the increased first-peak position in the ion-oxygen pair distribution function. In turn, the H-bond interactions among water molecules in the solvent become effectively strengthened. As a result, the population of interstitial water molecules increases in the first coordination shell, which are non-bonded to Cl- ion, however, are H-bonded to other water molecules in a tetrahedral network. By including NQEs, our local dielectric response function calculations suggest that the surrounding water molecules electronically screen the charge of Cl^- more effectively than the classical counterpart. Our resulting solvation structures are in excellent agreement with the recent neutron scattering experiments by Soper et al.