Molecular-Level Interpretation of Vibrational Spectra of Ordered Ice Phases.

We build on results from our previous investigation into ice Ih using a combination of classical many-body molecular dynamics (MB-MD) and normal mode (NM) calculations to obtain molecular-level information on the spectroscopic signatures in the OH stretching region for all seven of the known ordered crystalline ice phases. The classical MB-MD spectra are shown to capture the important spectral features by comparing with experimental Raman spectra. This motivates the use of the classical simulations in understanding the spectral features of the various ordered ice phases in molecular terms. This is achieved through NM analysis to first demonstrate that the MB-MD spectra can be well recovered through the transition dipole moments and polarizability tensors calculated from each NM. From the normal mode calculations, measures of the amount of symmetric and antisymmetric stretching are calculated for each ice, as well as an approximation of how localized each mode is. These metrics aid in viewing the ice phases on a continuous spectrum determined by their density. As in ice Ih, it is found that most of the other ordered ice phases have highly delocalized modes and their spectral features cannot, in general, be described in terms of molecular normal modes. The lone exception is ice VIII, the densest crystalline ice phase. Despite being found only at high pressure, the symmetry index shows a clear separation of symmetric and antisymmetric stretching modes, giving rise to two distinct features.