Cambridge Molecular Therapeutics Programme, Hutchison/MRC Research Centre, University of Cambridge, Hills Road, Cambridge, CB2 0XZ, United Kingdom.
J Chem Phys. 2012 Feb 14;136(6):064518. doi: 10.1063/1.3683447.
Water is one of the simplest molecules in existence, but also one of the most important in biological and engineered systems. However, understanding the structure and dynamics of liquid water remains a major scientific challenge. Molecular dynamics simulations of liquid water were performed using the water models TIP3P-Ewald, TIP4P-2005, TIP5P-Ewald, and SWM4-NDP to calculate the radial distribution functions (RDFs), the relative angular distributions, and the excess enthalpies, entropies, and free energies. In addition, lower-order approximations to the entropy were considered, identifying the fourth-order approximation as an excellent estimate of the full entropy. The second-order and third-order approximations are ~20% larger and smaller than the true entropy, respectively. All four models perform very well in predicting the radial distribution functions, with the TIP5P-Ewald model providing the best match to the experimental data. The models also perform well in predicting the excess entropy, enthalpy, and free energy of liquid water. The TIP4P-2005 and SWM4-NDP models are more accurate than the TIP3P-Ewald and TIP5P-Ewald models in this respect. However, the relative angular distribution functions of the four water models reveal notable differences. The TIP5P-Ewald model demonstrates an increased preference for water molecules to act both as tetrahedral hydrogen bond donors and acceptors, whereas the SWM4-NDP model demonstrates an increased preference for water molecules to act as planar hydrogen bond acceptors. These differences are not uncovered by analysis of the RDFs or the commonly employed tetrahedral order parameter. However, they are expected to be very important when considering water molecules around solutes and are thus a key consideration in modelling solvent entropy.
水是自然界中最简单的分子之一，但也是生物和工程系统中最重要的分子之一。然而，理解液态水的结构和动力学仍然是一个重大的科学挑战。使用 TIP3P-Ewald、TIP4P-2005、TIP5P-Ewald 和 SWM4-NDP 等水模型对液态水进行了分子动力学模拟，以计算径向分布函数（RDF）、相对角度分布以及过剩焓、熵和自由能。此外，还考虑了熵的低阶近似，确定四阶近似是对全熵的极好估计。二阶和三阶近似分别比真实熵大 20%和小 20%。所有四个模型在预测径向分布函数方面都表现得非常出色，其中 TIP5P-Ewald 模型与实验数据的拟合最好。这些模型在预测液态水的过剩熵、焓和自由能方面也表现良好。在这方面，TIP4P-2005 和 SWM4-NDP 模型比 TIP3P-Ewald 和 TIP5P-Ewald 模型更准确。然而，四个水模型的相对角度分布函数揭示了明显的差异。TIP5P-Ewald 模型表明水分子更倾向于同时作为四面体氢键供体和受体，而 SWM4-NDP 模型表明水分子更倾向于作为平面氢键受体。这些差异无法通过 RDF 或常用的四面体有序参数分析揭示。然而，在考虑溶质周围的水分子时，它们非常重要，因此是模拟溶剂熵的关键考虑因素。
