  There have been ample studies of the many phases of H2O in both its solid and
low pressure liquid states, and the transitions between them. Using molecular
dynamics simulations we address the hitherto unexplored deeply supercritical
pressures, where no qualitative transitions are thought to take place and where
all properties are expected to vary smoothly. On the basis of these simulations
we predict that water at supercritical pressures undergoes a structural
crossover across the Frenkel line at pressures as high as 45 times the critical
pressure. This provides a new insight into the water phase diagram and
establishes a link between the structural and dynamical properties of
supercritical water. Specifically, the crossover is demonstrated by a sharp and
pronounced at low pressures, and smooth at high pressures, signified by changes
in the pair distribution functions and local coordination which coincide with
the dynamical transition (the loss of all oscillatory molecular motion) at the
Frenkel line on the phase diagram.
