Ab initiostudies on the structural and dynamical properties of ice

The structural and dynamical properties of cubic ${\mathrm{H}}_2$O and ${\mathrm{D}}_2$O 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 ${\mathrm{I}}_{\mathit{c}}$), 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 ${\mathrm{I}}_{\mathit{c}}$, but it is found to occur at a pressure of 7 GPa at which ice ${\mathrm{I}}_{\mathit{c}}$ is unstable with respect to denser phases.