Molecular-Based Virial Coefficients of Pure Fluids and Mixtures

We report low-order virial coefficients for several systems of interest to engineering applications.  All values are computed using quantitative molecular models taken from the literature. The substances examined in this study are CO2, H2O, small alkanes, H2, and N2, as well as some mixtures formed from them. Calculations are based on either of two types of models: (1) fits to highly accurate ab initio calculations of the potential energy surfaces, and (2) semi-empirical force fields that have been fit to other experimental data. In various cases, consideration is given to the importance of nuclear quantum effects and methods to compute them, three-body contributions, and/or the ability of the derived coefficients to describe thermodynamic properties over a range of supercritical state conditions. All calculated values are compared to experimental data, where available. Comparison to experimentally-determined virial coefficients is a particularly useful means for validating the molecular models, as this can isolate multibody effects and, through the temperature dependence, help to pinpoint inaccurate components of the potential.