Dynamical properties of supercooled water close to the liquid-liquid coexistence lines, and their relation to those at ambient conditions.

We report results of molecular dynamics simulations of supercooled bulk water at 180 K, close to the liquid/liquid coexistence lines recently discovered in the supercooled regime, both experimentally and by computer simulations. Despite the considerable differences in the densities of the three states considered, the obtained velocity autocorrelation functions display very similar behaviour in every case. On the other hand, the corresponding spectra show the presence of three well-defined modes. The two modes at higher frequencies are assigned to the symmetric and asymmetric stretching motions, respectively, whereas the lowest frequency mode to the bending and torsional motions in clusters of five hydrogen-bonded water molecules. A careful fitting procedure demonstrates the presence of such clusters also in the normal liquid phase of water at ambient conditions.