  At 300-3000K and 1-500MPa, variations of relative contents for small water
clusters (H2O)n (n=1~6) were calculated by using statistical mechanical
methods. First, 9 kinds of small water clusters were selected and their
structures were optimized by using ab initio method. In the wide range of
temperature (300-3000K) and pressure (1-500MPa), their equilibrium constants of
reactions for formation of 9 kinds of water clusters were determined by using
molecular partition function. Next, changes of contents (molar fractions) as
function of temperature and pressure were estimated. The obtained results for
small water clusters can be used to interpret temperature-pressure dependency
of the average number for the hydrogen bonds in water clusters and
redistribution of the water clusters at the ultrasonic cavitation reactions.
