Takahashi Kazuaki Z
National Institute of Advanced Industrial Science and Technology (AIST), Research Center for Computational Design of Advanced Functional Materials, Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan.
J Chem Phys. 2024 Oct 7;161(13). doi: 10.1063/5.0205804.
Structures of liquid water are controversial not only in supercooled polyamorphism but also in stable bulk liquids in the high temperature and pressure range. Several experimental studies in bulk liquid have assumed the existence of three different liquid water structures. If indeed the three liquid water structures are different, they should be clearly distinguished by some measure other than density that characterizes the difference in structural order. In this study, whether the three different bulk liquid water structures are real or not is numerically verified based on molecular simulations using a reliable water molecular model. Since these liquid water structures have been suggested to be related to three different crystal structures (i.e., ice Ih, III, and V), liquid structures are sampled from the vicinity of the ice Ih-liquid coexistence point, the ice III-V-liquid triple point, and the ice V-VI-liquid triple point, respectively. An attempt is made to introduce local order parameters (LOPs) as an indicator to distinguish these structures. A fast and exhaustive LOP search is performed by the molecular assembly structure learning package for Identifying order parameters. The selected LOP distinguishes the molecular structures of three different stable liquid waters with high accuracy, providing numerical evidence that these structural orders differ from each other. Furthermore, regions of the liquid water structures are drawn on a phase diagram using the LOP, demonstrating their consistency with experimental studies.
液态水的结构不仅在过冷多晶型中存在争议，在高温高压范围内的稳定体相液体中也存在争议。一些关于体相液体的实验研究假定存在三种不同的液态水结构。如果这三种液态水结构确实不同，那么它们应该通过除密度之外的某种测量方法来清晰区分，这种测量方法能够表征结构有序性的差异。在本研究中，基于使用可靠水分子模型的分子模拟，对这三种不同的体相液态水结构是否真实存在进行了数值验证。由于这些液态水结构被认为与三种不同的晶体结构（即冰Ih、III和V）有关，因此分别从冰Ih - 液体共存点、冰III - V - 液体三相点以及冰V - VI - 液体三相点附近对液态结构进行采样。尝试引入局部有序参数（LOP）作为区分这些结构的指标。通过用于识别有序参数的分子组装结构学习软件包进行快速且详尽的LOP搜索。所选的LOP能够高精度地区分三种不同稳定液态水的分子结构，提供了这些结构有序性彼此不同的数值证据。此外，使用LOP在相图上绘制了液态水结构区域，证明了它们与实验研究的一致性。
