通过TIP4P/2005模型的计算机模拟获得的高压下水的phase diagram：塑性晶体相的出现
The phase diagram of water at high pressures as obtained by computer simulations of the TIP4P/2005 model: the appearance of a plastic crystal phase
摘要：
本文采用TIP 4P/2005水模型对水的高压区相图进行了计算机模拟研究。自由能计算进行冰VII和VIII和流体相，以确定这些冰的熔化曲线。此外，分子动力学模拟在高温（440 K）下进行，观察到在约80000巴的压力下液体自发冻结成固相。对所得结构的分析得出了形成塑性结晶相的结论。在塑性晶体阶段，氧原子排列形成一个体心立方结构，就像在冰VII中一样，但水分子几乎可以自由旋转。对这种新相进行了自由能计算，发现对于TIP 4P/2005，这种塑性晶体相在高于约400 K的温度下相对于冰VII和VIII是热稳定的，尽管精确值取决于压力。通过Gibbs Duhem模拟，确定了所有的共存线，得到了TIP 4P/2005模型的相图，包括冰VIII和冰VII以及新的塑性晶体相。TIP 4P/2005模型能够定性描述水的相图。研究这种塑性晶体相是否确实存在于真实的水中将是有趣的。水的近似球形使得在高温下形成塑性晶体相成为可能。从动力学的角度来看，在高温下形成塑性晶体相（具有氧原子的bcc排列）在约2ns内发生。这与冰Ih的成核相反，冰Ih的成核需要数百ns量级的模拟。
英文摘要：
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.
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