Temperature dependence of hydrogen bonding in supercritical water

The effect of temperature on hydrogen bonding in supercritical water is investigated at densities of 257 and 659 kg/m(3) and at temperatures in the range 773-1073 K using molecular dynamics simulations with a flexible simple point charge water potential. An energetic criterion is used to distinguish hydrogen-bonded pairs from non-hydrogen-bonded pairs. The number of hydrogen bonds per water molecule decreases as the temperature is increased. Hydrogen-bonded clusters in supercritical water consist of fewer than five members. Cluster size distributions are not strongly influenced by temperature at the higher density, although a slightly broader distribution is obtained at lower temperatures for the lower density. Hydrogen bond persistence time functions and autocorrelation functions exhibit faster decay at higher temperatures. The rupture of hydrogen bonds appears to be primarily temperature dependent, although the frequency of bond breakage is slightly higher at the higher density. The flexible water model exhibits slower hydrogen bond rupture rates than the corresponding rigid water model.