Molecular dynamics simulations of stretched water: Local structure and spectral signatures

Molecular dynamics simulations have been performed on a system of flexible extended simple point charge (SPC/E) model water molecules which include intramolecular stretching and bending terms. A series of densities, 0.70, 0.80, 0.90, and 1.00 g cm(-3) was simulated at 298 K, thereby investigating the early stages of bulk water ''stretching'' leading to cavitation. The local structural changes were followed using atom-resolved pair radial distribution functions which revealed that the local water structure deforms inhomogeneously as the density decreases below 1.00 g cm(-3). Snapshots of the configurations and the radial distribution functions revealed that the decrease in density was accommodated in the sample by the appearance of "cavities" coexisting with clusters of more bulklike water but which were increasingly strained as the density decreased. Power spectra in the region of 0-5000 cm(-1) were obtained from bond length, bond angle, and atom velocity autocorrelation functions. Combined with the power spectrum derived from the hydrogen atom velocity autocorrelation function, the vibrational frequencies of the gas and liquid states and the spectral features have been more clearly assigned. The power spectra are largely dominated by these near normal bulklike regions, which explains their observed insensitivity to density in the wide range studied. Another sequence of simulations was carried out at temperatures of 259, 273, and 350 K at a density of 1.00 g cm(-3). (C) 1998 American Institute of Physics. [S0021-9606(98)51621-2].