Making equation of state models predictive: Part 1: Quantum chemical computation of molecular properties

We perform quantum chemical calculations to determine molecular properties for a set of molecules for later use in equation of state applications. The properties are the dipole moment, the quadrupole moment, the dipole polarizability and the dipole–dipole dispersion coefficient. We study the effect of basis sets and quantum chemical levels on the accuracy of the computed properties. The aim is to find methods that provide a reasonable compromise between accuracy and effort for medium-sized molecules which are typical for equation of state applications. We assess the accuracy of a recent extrapolation method by Cybulski and Haley for the dispersion coefficient in combination with medium-sized basis sets to make it applicable for the molecules we are interested in. We also apply this approach to compute the deviation of the unlike-dispersion interaction coefficient from the geometric mean combination rule which we call dispersion non-ideality. We find that, due to cancellations of errors, the accuracy of the dispersion non-ideality is much higher than that of the calculation of absolute C(6) coefficients.