Theoretical study of the properties of low-frequency vibrations of low, high, and very high density amorphous ices

Model structures of low, high, and very high density amorphous ices are obtained by the molecular dynamics and conjugate gradient methods. By means of the lattice dynamics method the vibrational properties of the obtained structures of amorphous ices are calculated. The delocalization of collective vibrations in amorphous ices is shown. The distribution of the number of molecules over the potential energies and amplitudes is derived. The directions of vector projections of the instantaneous displacement of molecules reveal the similarity of low-frequency collective vibrations to optical vibrations in crystals. Model cells of amorphous ices are considered as the crystal unit cells having periodical boundary conditions. Based on this assumption, the dispersion curves of amorphous ices are constructed, which also indicate the crystal-like collective vibrations.