Symmetry classification of the projected vibrational density of states in ice VIII from ab initio methods

We have determined the normal modes of ice VIII, a 16-molecule supercell, using ab initio calculations. The dynamical matrices so obtained are diagonalized and the eigenvectors projected on to the stretching, bending and Libration modes of each water molecule in turn. We can therefore accurately correlate the symmetry assignments of the zone centre modes and the corresponding frequencies of these dynamical results with Raman, infrared and neutron scattering data. In particular, using neutron scattering data, we obtain excellent agreement with the frequencies of the v(Ry)B(Iu) phonon mode which is inactive in Raman or infrared spectroscopy.