Thermodynamics Research Center, National Institute of Standards and Technology, Boulder, Colorado 80305, USA.
Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309, USA.
J Chem Phys. 2018 Sep 21;149(11):114109. doi: 10.1063/1.5039504.
Molecular simulation results at extreme temperatures and pressures can supplement experimental data when developing fundamental equations of state. Since most force fields are optimized to agree with vapor-liquid equilibria (VLE) properties, however, the reliability of the molecular simulation results depends on the validity/transferability of the force field at higher temperatures and pressures. As demonstrated in this study, although state-of-the-art united-atom Mie -6 potentials for normal and branched alkanes provide accurate estimates for VLE, they tend to over-predict pressures for dense supercritical fluids and compressed liquids. The physical explanation for this observation is that the repulsive barrier is too steep for the "optimal" united-atom Mie -6 potential parameterized with VLE properties. Bayesian inference confirms that no feasible combination of non-bonded parameters (, , and ) is capable of simultaneously predicting saturated vapor pressures, saturated liquid densities, and pressures at high temperatures and densities. This conclusion has both practical and theoretical ramifications, as more realistic non-bonded potentials may be required for accurate extrapolation to high pressures of industrial interest.
在开发基本状态方程时，极端温度和压力下的分子模拟结果可以补充实验数据。然而，由于大多数力场都是经过优化以与汽液平衡（VLE）性质一致，因此分子模拟结果的可靠性取决于力场在更高温度和压力下的有效性/可转移性。正如本研究所示，尽管用于正构和支链烷烃的最先进的统一原子 Mie-6 势能够准确估计 VLE，但它们往往会过高预测稠密超临界流体和压缩液体的压力。这种观察结果的物理解释是，对于用 VLE 性质参数化的“最佳”统一原子 Mie-6 势能，排斥势垒过于陡峭。贝叶斯推断证实，没有可行的非键参数（、、和）组合能够同时预测饱和蒸气压、饱和液体密度以及高温和高密度下的压力。这一结论既有实际意义，也有理论意义，因为对于准确外推到工业感兴趣的高压，可能需要更现实的非键势。
