  Because of the negative inclination of the solid-liquid phase separation line
in water, ice Ih melts on compression. On further increase in pressure the
liquid water transforms into a high density metastable glassy state,
characterized by a rapid approach to zero diffusion coefficient and an absence
of any crystalline order in the static structure factor. The vitrification is
found to occur even at high temperatures (T > 250 K). We study this glass
transition process at four temperatures (80 K, 250 K, 300 K and 320 K). The
transition pressure increases with increase in temperature, as expected.
Interestingly, we find that the total entropy of the system exhibits a sharp
crossover near the glass transition pressure where the diffusion of water goes
to zero. The diffusion coefficient shows an exponential dependence on the
properly defined excess entropy. In an interesting result not reported before,
we find a pressure induced realignment of water molecules resulting in two well
separated peaks in the O-O-O angle distribution among neighbouring molecules.
The difference between the positions of these two peaks undergoes a sharp
change at the vitrification pressure suggesting that it can serve as an
appropriate order parameter to detect the glass transition point.
