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Since the observation of two intermolecular translational optic peaks at 28 and 37 meV using inelastic neutron scattering techniques, there has been a renewed effort to reproduce phonon density of states with these peaks by computer simulation. In this paper, we demonstrate our attempt to calculate the measured inelastic neutron spectrum using molecular dynamic techniques for a number of potentials, such as TIP4P, MCY, and KKY potentials. The results show that none of them can reproduce the double peaks seen in the measured spectrum.

Incoherent inelastic neutron-scattering spectra of ice VII and VIII have been obtained at an in situ pressure of 2.2 ± 0.1 GPa on the TFXA spectrometer at the pulsed neutron source, ISIS. Comparisons with recovered phases (at ambient pressure) show large shifts of the phonon modes. For instance, the optic peak for ice VIII is shifted from 25 to 32 meV (dω/ω = 0.20 ± 0.01). The coefficient of increase of the frequency of the mode with pressure, dω/dp, is 2.3 meV/GPa (or 18.5 cm −1 /GPa). In ice VII, the proton-disordered form of ice VIII, the spectra, measured under the same pressure, show that: firstly, the sharp features of the librational band in the region 50-100 meV disappear, indicating that the protons are indeed disordered as is found in other proton-disordered forms of ice; secondly, a weak higher-energy optic mode at 49.5 meV appears due to the presence of the second proton configuration as seen in other proton-disordered ice Ic.