Calculations of the lattice mode spectra of proton ordered ices: A test of the accuracy of water–water potentials

We report the results of calculations of the lattice mode spectra of ices II, VIII, and IX, designed to test the accuracy of several water–water potentials. The potentials which give the best (but not accurate) descriptions of the lattice structures, densities, and energies of these ices are found to give the proper overall description of the distribution of their lattice vibrations and the changes in frequency induced by isotopic substitution. Just as none of the available potentials gives accurate predictions of the lattice densities and energies, none gives a quantitative description of the lattice spectra of these ices. Nevertheless, there are some features of the calculations for the several water–water potentials which are robust and likely correct for the real systems. The most important of these predictions is that there is relatively little translation-libration coupling in the proton ordered ices.