(508d) Computing Virial Coefficients to Assess the Accuracy of Intermolecular Potentials

The most widely used molecular models are pairwise-additive and have been formulated to describe properties of condensed phases. As a result they do not describe the true interactions between molecule pairs, but rather are effective potentials that attempt to include multibody interactions and other temperature- and density-dependent effects in the pairwise form. Some models include polarization, which allows the pair interactions to be described more accurately at the expense of increased computational cost. Model parameters are typically fit to experimental data such as vapor-liquid equilibria, P-V-T etc. Since B_n is a direct function of the interaction of n molecules, fitting to experimental virial coefficient data may yield a model which more accurately describes those interactions. Model virial coefficients can also be a route to inexpensively generate non-condensed P-V-T data from the virial equation of state (VEOS).

In this paper, we describe a systematic, comprehensive effort to evaluate high-order virial coefficients along with their temperature derivatives. We consider here in particular TraPPE potentials for a collection of 10 species, and some mixtures formed from them. We apply a recently developed algorithm that facilitates calculation of the coefficients and their derivatives for multibody potentials. Knowledge of the derivatives is needed for certain properties and also allows us to describe a wider temperature range while sampling only a few temperatures. The coefficients can be used to formulate an approximant which converges up to higher densities as compared to the VEOS. Our larger aim is to collect these results in a database to create a publicly accessible resource of coefficients that can be used to compute all thermodynamic properties of these mixtures in the vapor and supercritical-fluid phases. In this manner the accuracy of a molecular model can be assessed and its usefulness greatly extended.