Process and Energy Laboratory, Delft University of Technology, Leeghwaterstraat 44, 2628CA Delft, The Netherlands.
J Phys Chem B. 2011 Jun 23;115(24):7872-80. doi: 10.1021/jp2026219. Epub 2011 May 31.
Force field parameters used in classical molecular simulations can be estimated from quantum mechanical calculations or spectroscopic measurements. This especially applies to bonded interactions such as bond-stretching, bond-bending, and torsional interactions. However, it is difficult and computational expensive to obtain accurate parameters describing the nonbonded van der Waals interactions from quantum mechanics. In many studies, these parameters are adjusted to reproduce experimental data, such as vapor-liquid equilibria (VLE) data. Adjusting these force field parameters to VLE data is currently a cumbersome and computationally expensive task. The reason is that the result of a calculation of the vapor-liquid equilibria depends on the van der Waals interactions of all atom types in the system, therefore requiring many time-consuming iterations. In this work, we use an analytical equation of state, the perturbed chain statistical associating fluid theory (PC-SAFT), to predict the results of molecular simulations for VLE. The analytical PC-SAFT equation of state is used to approximate the objective function f(p) as a function of the array of force field parameters p. The objective function is here for example defined as the deviations of vapor pressure, enthalpy of vaporization and liquid density data, with respect to experimental data. The parameters are optimized using the analytical PC-SAFT equation of state, which is orders of magnitude quicker to calculate than molecular simulation. The solution is an excellent approximation of the real objective function, so that the resulting method requires only very few molecular simulation runs to converge. The method is here illustrated by optimizing transferable Lennard-Jones parameters for the n-alkane series. Optimizing four force field parameters p = (ε(CH(2))(CH(2)), ε(CH(3))(CH(3)), σ(CH(2))(CH(2)), σ(CH(3))(CH(3))) we obtain excellent agreement of coexisting densities, vapor pressure and caloric properties within only 2 -3 molecular simulation runs.
经典分子模拟中使用的力场参数可以从量子力学计算或光谱测量中估算得到。这尤其适用于键合相互作用，如键拉伸、键弯曲和扭转相互作用。然而，从量子力学中获得准确描述非键范德华相互作用的参数是困难且计算昂贵的。在许多研究中，这些参数被调整以重现实验数据，例如汽液平衡（VLE）数据。将这些力场参数调整到 VLE 数据目前是一项繁琐且计算昂贵的任务。原因是汽液平衡计算的结果取决于系统中所有原子类型的范德华相互作用，因此需要许多耗时的迭代。在这项工作中，我们使用分析状态方程，即受扰链统计关联流体理论（PC-SAFT），来预测 VLE 的分子模拟结果。分析 PC-SAFT 状态方程用于将目标函数 f(p)近似为力场参数 p 的数组的函数。这里的目标函数例如定义为蒸气压、汽化焓和液体密度数据相对于实验数据的偏差。使用分析 PC-SAFT 状态方程对参数进行优化，该方程的计算速度比分子模拟快几个数量级。该解决方案是真实目标函数的极好近似，因此该方法仅需要很少的分子模拟运行即可收敛。该方法通过优化 n-烷烃系列的可转移 Lennard-Jones 参数来说明。通过优化四个力场参数 p = (ε(CH(2))(CH(2))，ε(CH(3))(CH(3))，σ(CH(2))(CH(2))，σ(CH(3))(CH(3)))，我们仅通过 2-3 次分子模拟运行即可获得极好的共存密度、蒸气压和热性质的一致性。
