Department of Fundamental Engineering, Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan.
J Chem Phys. 2018 Mar 28;148(12):124503. doi: 10.1063/1.5024565.
The microscopic structure of liquid water has been believed to be the key to the understanding of the unique properties of this extremely important substance. Many structural descriptors have been developed for revealing local structural order in water, but their properties are still not well understood. The essential difficulty comes from structural fluctuations due to thermal noise, which are intrinsic to the liquid state. The most popular and widely used descriptors are the local structure index (LSI) and d. Recently, Russo and Tanaka [Nat. Commun. 3, 3556 (2014)] introduced a new descriptor ζ which measures the translational order between the first and second shells considering hydrogen bonding (H-bonding) in the first shell. In this work, we compare the performance of these three structural descriptors for a popular water model known as TIP5P water. We show that local structural ordering can be properly captured only by the structural descriptor ζ, but not by the other two descriptors particularly at a high temperature, where thermal noise effects are severe. The key difference of ζ from LSI and d is that only ζ considers H-bonding which is crucial to detect high translational and tetrahedral order of not only oxygen but also hydrogen atoms. The importance of H-bonding is very natural, considering the fact that the locally favored structures are stabilized by energy gain due to the formation of four hydrogen bonds between the central water molecule and its neighboring ones in the first shell. Our analysis of the water structure by using ζ strongly supports the two-state model of water: water is a dynamic mixture of locally favored (ordered) and normal-liquid (disordered) structures. This work demonstrates the importance of H-bonding in the characterization of water's structures and provides a useful structural descriptor for water-type tetrahedral liquids to study their structure and dynamics.
液态水的微观结构被认为是理解这种极其重要物质独特性质的关键。已经开发出许多结构描述符来揭示水中的局部结构有序性，但它们的性质仍未得到很好的理解。基本的困难来自于热噪声引起的结构波动，这是液体状态的固有特性。最流行和广泛使用的描述符是局部结构指数（LSI）和 d。最近，Russo 和 Tanaka [Nat. Commun. 3, 3556 (2014)] 引入了一个新的描述符 ζ，它考虑了第一层的氢键（H 键），用于测量第一层和第二层之间的平移有序性。在这项工作中，我们比较了这三个结构描述符在一种流行的称为 TIP5P 水的水模型中的性能。我们表明，只有结构描述符 ζ 才能正确捕捉局部结构有序性，而其他两个描述符特别是在高温下不能，因为高温下热噪声的影响很严重。 ζ 与 LSI 和 d 的关键区别在于，只有 ζ 考虑了 H 键，这对于检测不仅氧原子而且氢原子的高平移和四面体有序性至关重要。考虑到由于中心水分子与其第一层相邻水分子之间形成四个氢键而导致的能量增益，局部有利结构得到稳定，因此 H 键的重要性是非常自然的。我们通过使用 ζ 对水结构的分析强烈支持水的两态模型：水是局部有利（有序）和正常液体（无序）结构的动态混合物。这项工作表明 H 键在水结构的表征中的重要性，并为水型四面体液体提供了一个有用的结构描述符，以研究它们的结构和动力学。
