  In this work the high pressure region of the phase diagram of water has been
studied by computer simulation by using the TIP4P/2005 model of water. Free
energy calculations were performed for ices VII and VIII and for the fluid
phase to determine the melting curve of these ices. In addition molecular
dynamics simulations were performed at high temperatures (440K) observing the
spontaneous freezing of the liquid into a solid phase at pressures of about
80000 bar. The analysis of the structure obtained lead to the conclusion that a
plastic crystal phase was formed. In the plastic crystal phase the oxygen atoms
were arranged forming a body center cubic structure, as in ice VII, but the
water molecules were able to rotate almost freely. Free energy calculations
were performed for this new phase, and it was found that for TIP4P/2005 this
plastic crystal phase is thermodynamically stable with respect to ices VII and
VIII for temperatures higher than about 400K, although the precise value
depends on the pressure. By using Gibbs Duhem simulations, all coexistence
lines were determined, and the phase diagram of the TIP4P/2005 model was
obtained, including ices VIII and VII and the new plastic crystal phase. The
TIP4P/2005 model is able to describe qualitatively the phase diagram of water.
It would be of interest to study if such a plastic crystal phase does indeed
exist for real water. The nearly spherical shape of water makes possible the
formation of a plastic crystal phase at high temperatures. The formation of a
plastic crystal phase at high temperatures (with a bcc arrangements of oxygen
atoms) is fast from a kinetic point of view occurring in about 2ns. This is in
contrast to the nucleation of ice Ih which requires simulations of the order of
hundreds of ns.
