Molecular Simulation of Caloric Properties of Fluids Modelled by Force Fields with Intramolecular Contributions: Application to Heat Capacities

Abstract

The calculation of caloric properties such as heat capacity, Joule-Thomson coefficients and the speed of sound by classical force-field-based molecular simulation methodology has received scant attention in the literature, particularly for systems composed of complex molecules whose force fields (FFs) are characterized by a combination of intramolecular and intermolecular terms (referred to herein as "flexible FFs"). The calculation of a thermodynamic property for a system whose molecules are described by such a FF involves the calculation of the residual property prior to its addition to the corresponding ideal-gas (IG) property, the latter of which is separately calculated, either using thermochemical compilations or nowadays accurate quantum mechanical calculations. Although the simulation of a volumetric residual property proceeds by simply replacing the intermolecular FF in the rigid molecule case by the total (intramolecular plus intermolecular) FF, this is not the case for a caloric property. We discuss the methodology required in performing such calculations, and focus on the example of the molar heat capacity at constant pressure, $c_P$, one of the most important caloric properties. We also consider three approximations for the calculation procedure, and illustrate their consequences for the examples of the relatively simple molecule 2-propanol, ${\rm CH_3CH(OH)CH_3}$, and for monoethanolamine, ${\rm HO(CH_2)_2NH_2}$, an important fluid used in carbon capture.