Nature and stability of ice X

A study of pressure effects on the vibrational frequencies of ice X is performed to shed light on the existence and stability of this phase. The analysis reveals: (i) its stability range, (ii) the soft phonon nature of the transition at high pressures to the Pbcm structure proposed by Benoit et al. (M. Benoit, M. Bernasconi, P. Focher, and M. Parrinello, New high-pressure phase of ice, Phys. Rev. Lett. 76 (1996), pp. 2934-2936.) and (iii) the phonon collapse associated with the dynamical disordered structure at low pressures. Additionally, a topological analysis of the electron localization function and the electron density through the atoms-in-molecules formalism clarifies the chemical nature of ice X.