ICTP East Africa Institute for Fundamental Research, University of Rwanda, Rwanda.
Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA.
J Chem Phys. 2019 Feb 28;150(8):084501. doi: 10.1063/1.5083826.
Molecular dynamics (MD) simulations of methane-water mixtures were performed using ab initio force fields for the CH-HO, HO-HO, and CH-CH interactions. Both methane and water molecules were polarizable. From these calculations, the potential of mean force (PMF) between two methane molecules was extracted. Our results are compared with PMFs from a density-functional-theory (DFT) based Born-Oppenheimer type MD (BOMD) simulation, from a Monte Carlo (MC) simulation with ab initio-based force fields, and from MD simulations with empirical force fields. Our PMF is qualitatively similar to that obtained from the simulations with empirical force fields but differs significantly from those resulting from the DFT-BOMD and MC simulations. The depth of the PMF global minimum obtained in the present work is in a much better agreement with the experimental estimate than the result of the DFT-BOMD simulation, possibly due to the inability of DFT to describe the dispersion interactions and the lack of extensive sampling in the BOMD simulations. Our work indicates that, for a pair of methane molecules, there are configurations where the solvent increases the attraction between the solutes, but there are also conformations in which the solvent causes a weak net repulsion. On average, the methane molecules are more likely to be in the configuration where they are separated by a water molecule than in the one in which they are in contact even though the minimum of the PMF at the latter configuration is deeper than that at the former. Finally, we found that the water structure around methane solutes does not show a greater tetrahedral ordering than in neat bulk water.
采用 CH-HO、HO-HO 和 CH-CH 相互作用的从头算力场对甲烷-水混合物进行了分子动力学（MD）模拟。甲烷和水分子都是极化的。从这些计算中，提取了两个甲烷分子之间的平均力势（PMF）。我们的结果与基于密度泛函理论（DFT）的 Born-Oppenheimer 型 MD（BOMD）模拟、基于从头算力场的蒙特卡罗（MC）模拟和经验力场的 MD 模拟的 PMF 进行了比较。我们的 PMF 在定性上与经验力场模拟得到的 PMF 相似，但与 DFT-BOMD 和 MC 模拟得到的 PMF 有显著差异。与 DFT-BOMD 模拟结果相比，本工作中得到的 PMF 全局最小值的深度与实验估计值更吻合，这可能是由于 DFT 无法描述色散相互作用以及 BOMD 模拟中缺乏广泛采样。我们的工作表明，对于一对甲烷分子，存在溶剂增加溶质之间吸引力的构型，但也存在溶剂导致弱净排斥的构象。平均而言，甲烷分子更有可能处于被水分子隔开的构型，而不是处于相互接触的构型，尽管后一种构型的 PMF 最小值比前一种构型的深。最后，我们发现甲烷溶剂周围的水分子结构并没有比纯水中表现出更大的四面体有序性。
