Phase Diagrams of TIP4P/2005, SPC/E, and TIP5P Water at High Pressure

We investigate high-pressure ice phases using molecular dynamics simulations. Spontaneous nucleation of a new crystalline solid, named ice T2, is observed in a simulation of TIP4P/2005 water at 260 K and 3.3 GPa. The phase diagram of ices VI, VII, T2, and recently reported two other hypothetical ices, ice R and ice T, is determined using the direct coexistence method and the Clausius-Clapeyron equation for TIP4P/2005, SPC/E, and TIP5P water. It is found that there exists at least one pressure region in which a hypothetical ice phase is the most stable at ambient temperature with those models. Although the hypothetical ices may be metastable in reality, these ices could be of great importance toward a comprehensive understanding of the phase behaviors of water including many metastable ice polymorphs settled in the hidden area of T- P space. The unit cell of ice T2 is tetragonal with a space group of I4₁/ acd and it contains 152 water molecules. This is probably the most entangled structure among crystals which have been found in nucleation simulations without bias.