Wierzchowski S J, Monson P A
Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 01003, USA.
J Phys Chem B. 2007 Jun 28;111(25):7274-82. doi: 10.1021/jp068325a. Epub 2007 May 26.
We describe a method for calculating free energies and chemical potentials for molecular models of gas hydrate systems using Monte Carlo simulations. The method has two components: (i) thermodynamic integration to obtain the water and guest molecule chemical potentials as functions of the hydrate occupancy; (ii) calculation of the free energy of the zero-occupancy hydrate system using thermodynamic integration from an Einstein crystal reference state. The approach is applicable to any classical molecular model of a hydrate. We illustrate the methodology with an application to the structure-I methane hydrate using two molecular models. Results from the method are also used to assess approximations in the van der Waals-Platteeuw theory and some of its extensions. It is shown that the success of the van der Waals-Platteeuw theory is in part due to a cancellation of the error arising from the assumption of a fixed configuration of water molecules in the hydrate framework with that arising from the neglect of methane-methane interactions.
我们描述了一种使用蒙特卡罗模拟来计算气体水合物系统分子模型的自由能和化学势的方法。该方法有两个组成部分：(i) 热力学积分，以获得作为水合物占有率函数的水和客体分子的化学势；(ii) 使用从爱因斯坦晶体参考态进行的热力学积分来计算零占有率水合物系统的自由能。该方法适用于任何水合物的经典分子模型。我们用两个分子模型对结构-I甲烷水合物的应用来说明该方法。该方法的结果还用于评估范德华-普拉特尤理论及其一些扩展中的近似值。结果表明，范德华-普拉特尤理论的成功部分归因于水合物框架中水分子固定构型假设所产生的误差与甲烷-甲烷相互作用被忽略所产生的误差相互抵消。
