AB-INITIO MOLECULAR-DYNAMICS STUDY OF CRYSTAL HYDRATES OF HCL INCLUDING PATH-INTEGRAL RESULTS

The hydrates of hydrogen chloride are ionic crystals, which contain hy dronium. The hydronium in the monohydrate has been reported to be stat istically disordered between two possible sites related by inversion s ymmetry. Ab initio molecular dynamics (MD) calculations are presented for the mono-, di- and tri-hydrates of hydrogen chloride using the den sity functional based Car-Parrinello technique. The simulations were c arried out with the goal of investigating proton disorder in these cry stals. The possible role of nuclear quantum effects has been explored via path integral MD simulations. The results suggest that the propose d disordered sites in the monohydrate are dynamically unstable and the refore unlikely to be responsible for the reported disorder. Unfortuna tely, little information was obtained for the dihydrate because the la rge unit cell leads to difficulties in carrying out the simulations. N uclear quantum effects are shown to be important for characterizing th e proton distributions in the trihydrate.