MOLECULAR-DYNAMICS SIMULATIONS OF STRETCHED WATER - LOCAL-STRUCTURE AND SPECTRAL SIGNATURES

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