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

水在热容和等温压缩率等热力学响应函数中表现出各种异常，在过冷状态下趋于发散。从分子动力学（MD）模拟中，Stanley和同事已经解释了这一点，引入了共存线的第二个临界点，在该临界点之上，低密度和高密度非晶态冰（LDA和HDA）在温度-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它们所定位的临界点相同的压力范围内失去异常的事实相协调。Speedy推测LDA在大气压下没有从正常水开始的连续的热力学路径。这与MD模拟研究结果形成鲜明对比，MD模拟研究得出的结论是不存在相边界，水是连续变化的。我们在恒压下进行了长时间的MD模拟，发现在213k左右热力学和结构性质发生了突变。这一发现使我们能够调和两个观点，第二个临界点和水II。研究了过冷水两相中不完全配位水分子的配位数波动和空间相关性。结果表明，虽然两相局部能量最小结构的平均配位数均为4，但两相间的配位数波动幅度有显著差异。研究了一邻和二邻完全（四）配位的水分子的连通性。在低密度液相中，这种局部有序的水分子分布在整个体系中，而在高密度液相中，这些水分子仅形成小尺寸的团簇。两种水相的连通性不同，两种液体状态之间的相变可以重新解释为局部有序水分子的渗透。

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.