Theoretical study of the isostructural transformation in ice VIII

We present a first-principles study of the structural and vibrational properties of ${\mathrm{D}}_2\mathrm{O}$-ice VIII. We do not confirm the existence of the structural discontinuity that was proposed on the basis of a neutron diffraction study and of a first principles calculation, although we reproduce well other physical properties, including the nonlinear behavior of phonon frequencies below $3\mathrm{GPa}$. This study brings about the necessity of further experiments to investigate the structure of ice VIII at high pressure. The relationship between phonon frequencies and structural parameters is clarified in detail.