Ab initio molecular dynamics study of crystal hydrates of HCl including path integral results

The hydrates of hydrogen chloride are ionic crystals, which contain hydronium. The hydronium in the monohydrate has been reported to be statistically disordered between two possible sites related by inversion symmetry. Ab initio molecular dynamics (MD) calculations are presented for the mono-, di- and tri-hydrates of hydrogen chloride using the density functional based Car-Parrinello technique. The simulations were carried out with the goal of investigating proton disorder in these crystals. The possible role of nuclear quantum effects has been explored via path integral MD simulations. The results suggest that the proposed disordered sites in the monohydrate are dynamically unstable and therefore unlikely to be responsible for the reported disorder. Unfortunately, little information was obtained for the dihydrate because the large unit cell leads to difficulties in carrying out the simulations. Nuclear quantum effects are shown to be important for characterizing the proton distributions in the trihydrate.