Theoretical Study on a phase transition between low temperature metastable water phases

水的热力学响应函数如热容、等温压缩系数等在过冷状态下表现出各种异常，并趋于发散。从分子动力学（MD）模拟，这已占斯坦利和同事，介绍了第二个临界点的共存线以上的低和高密度的无定形冰（LDA和HDA）的温度-压力平面上分离。然而，目前尚不清楚如何将临界行为与水在与临界点相同的压力范围内失去其异常的事实相协调。斯皮蒂证实，在大气压下，LDA没有从正常水开始的化学连续路径。这是在尖锐的sontrast到MD模拟研究，其中得出的结论是，没有相边界存在和水不断变化。我们已经进行了长时间的MD模拟在恒定的压力下，发现在213 K左右的热力学和结构性质的突然变化。这一发现使我们能够调和这两个想法，第二临界点和水II。我们调查的波动的协调数和不完全协调的水分子在两个阶段的过冷水的空间相关性。结果表明，虽然两相的局部能量极小结构中平均配位数均为4，但涨落的大小在两相之间有很大的差异。水分子的第一和第二邻居也完全（四）协调的连接性进行检查。结果表明，这种局部有序的水分子在低密度液相中遍布整个系统，而这些分子在高密度液态水相中仅形成小尺寸的簇。两种水相的连通性不同，两种液态之间的相变可归结为局部有序水分子的渗流。

Water exhibits various anomalies in thermodynamic response functions such as heat capacity and isothermal compressibility, which tend to diverge in supercooled state. From molecular dynamics (MD) simulation, this has been accounted for by Stanley and coworkers, introducing a second critical point of the coexistence line above which low and high density amorphous ices (LDA and HDA) are separated on temperature-pressure plane. It is, however, not clear how to reconcile the critical behaviors with the fact that water loses its anomalies around the same pressure range as the critical point was located by them. Speedy conjectured that LDA has no thermodynamically continuous path from normal water at atmospheric pressure. This is in sharp sontrast to MD simulation study, where it is concluded that no phase boundary exists and water changes continuously. We have performed long MD simulations at constant pressure and found abrupt changes in thermodynamic and structural properties around 213 K.This finding enables us to reconcile both ideas, the second critical point and water II.We investigate the fluctuations of coordination numbers and the spatial correlations of imperfectly coordinated water molecules in two phases of supercooled water. It is found, although the average coordination numbers are 4 in local energy minimum structures of both phases, the magnitude of the fluctuation differs significantly between two phases. Connectivity of water molecules whose first and second neighbors are also perfectly (four-) coordinated is examined. It is revealed that such locally ordered water molecules spread over the entire system in the low density liquid phase whereas those molecules form only small size clusters in the high density liquid water phase. The two water phases are different in conncstivity and the phase transition between two liquid states can be recast into the percolation of locally ordered water molecules.