ABINITIO STUDIES ON THE STRUCTURAL AND DYNAMIC PROPERTIES OF ICE

The structural and dynamical properties of cubic H2O and D2O ice phases are studied using ab initio molecular dynamics combined with ultrasoft pseudopotentials. Phonon frequencies are extracted from the velocity autocorrelation functions; contributions from different normal modes in the phonon spectra are separated and easily identified. For the low-pressure phases, the agreement with the experimental data is reasonable and the isotope effects are well reproduced. High-pressure phases are also studied. The equations of state for cubic ice (ice I(c)), and for the ice VII-VIII-X family, are calculated. It is found that the local-density approximation must be augmented with gradient corrections in order to obtain a proper description of the hydrogen bond. Finally, the hydrogen-bond symmetrization, which is responsible for the transition from ice VII-VIII to ice X, is studied and is predicted to occur at 49 GPa. The nature of the phase transition is found to be that of a mode-softening transition. The corresponding symmetrization is also studied in ice I(c), but it is found to occur at a pressure of 7 GPa at which ice I(c) is unstable with respect to denser phases.