Wikfeldt Kjartan T, Leetmaa Mikael, Ljungberg Mathias P, Nilsson Anders, Pettersson Lars G M
FYSIKUM, AlbaNova University Center, Stockholm University, SE-106 91 Stockholm, Sweden.
J Phys Chem B. 2009 May 7;113(18):6246-55. doi: 10.1021/jp9007619.
We use the reverse Monte Carlo (RMC) method to critically evaluate the structural information content of diffraction data on bulk water by fitting simultaneously or separately to X-ray and neutron data; the O-H and H-H, but not the O-O, pair-correlation functions (PCFs) are well-described by the neutron data alone. Enforcing at the same time different H-bonding constraints, we generate four topologically different structure models of liquid water, including a simple mixture model, that all equally well reproduce the diffraction data. Although earlier work [Leetmaa, M.; et al. J. Chem. Phys. 2008, 129, 084502] has focused on tetrahedrality in the H-bond network in liquid water, we show here that, even for the O-O-O three-body correlation, tetrahedrality is not strictly defined by the data. We analyze how well two popular MD models (TIP4P-pol2 and SPC/E) reproduce the neutron data in q-space and find differences in important aspects from the experiment. From the RMC fits, we obtain pair-correlation functions (PCFs) that are in optimal agreement with the diffraction data but still show a surprisingly strong variability both in position and height of the first intermolecular (H-bonding) O-H peak. We conclude that, although diffraction data impose important constraints on the range of possible water structures, additional data are needed to narrow the range of possible structure models.
我们使用逆蒙特卡罗（RMC）方法，通过同时或分别拟合X射线和中子数据，来严格评估大块水体衍射数据的结构信息含量；仅中子数据就能很好地描述O-H和H-H对关联函数（PCF），但无法描述O-O对关联函数。同时施加不同的氢键约束，我们生成了四种拓扑结构不同的液态水结构模型，包括一个简单混合模型，所有这些模型都能同样好地重现衍射数据。尽管早期工作[Leetmaa, M.; 等人，《化学物理杂志》，2008年，129卷，084502期]关注的是液态水中氢键网络的四面体性，但我们在此表明，即使对于O-O-O三体相关性，数据也并未严格定义四面体性。我们分析了两种流行的分子动力学（MD）模型（TIP4P-pol2和SPC/E）在q空间中重现中子数据的程度，并发现与实验在重要方面存在差异。通过RMC拟合，我们获得了与衍射数据最佳吻合的对关联函数（PCF），但第一个分子间（氢键）O-H峰的位置和高度仍显示出惊人的强烈变化。我们得出结论，尽管衍射数据对可能的水结构范围施加了重要约束，但仍需要额外的数据来缩小可能的结构模型范围。
