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

水在热容、等温压缩性等热力学响应函数上表现出各种异常，在过冷状态下容易发散。从分子动力学(MD)模拟来看，Stanley和他的同事们解释了这一点，在共存线上引入了第二个临界点，在该临界点之上，低密度和高密度无定形冰(LDA和HDA)在温度-压力平面上被分离。然而，目前尚不清楚如何将临界行为与水在与临界点所处的压力范围相同的压力范围内失去异常的事实相协调。斯皮蒂推测，LDA在大气压下没有从正常水到热力学上连续的路径。这与MD模拟研究形成鲜明对比，在MD模拟研究中，得出的结论是不存在相界，水不断变化。我们在恒压下进行了长时间的MD模拟，发现在213K附近热力学和结构性质发生了突变。这一发现使我们能够调和第二临界点和水这两个概念。我们研究了过冷水两相中配位数的涨落和非完全配位水分子的空间关联。结果表明，虽然两相的局域能量极小结构的平均配位数均为4，但两相之间的起伏幅度有很大差异。研究了水分子的连通性，这些水分子的第一和第二邻居也是完全(四)配位的。结果表明，在低密度液相中，这种局部有序水分子分布在整个体系中，而在高密度液相中，这些分子只形成小尺寸的团簇。这两个水相的连接性不同，两个液态之间的相变可以重塑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.