Structure and OH-stretch spectroscopy of low- and high-density amorphous ices

We use the E3B water simulation model (which explicitly includes three-body interactions) and molecular dynamics simulations to study the structure of low- and high-density amorphous ices. We find that the "interstitial" molecule in high-density amorphous ice, which is responsible for the higher density, is not hydrogen bonded to the central molecule or its nearest neighbors. This molecule enforces a wider range of local environments as demonstrated by the tetrahedral order parameter, whereas the local structure of low-density amorphous ice is more ordered. We also present theoretical vibrational spectra (infrared, Raman, and two-dimensional infrared) for the amorphous phases, for both HOD/D2O and H2O. The results are in qualitative agreement with experiment and reflect the fact that low-density amorphous ice is more ordered, with stronger hydrogen bonds, compared to high-density amorphous ice. For both the structural analysis and spectral calculations, we compare the results for the amorphous ices with those for crystalline ice Ih and supercooled water.