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. (c) 2007 Elsevier B.V. All rights reserved.