A molecular model for orientational ordering in liquid crystal phases is presented. A van der Waals picture is adopted, with repulsive and attractive interactions described, respectively, in terms of excluded volume and surface contact interactions for pairs of molecules. The model, based on a realistic representation of the molecular structure, is applied to the nematic phase of para-azoxyanisole (PAA). Temperature and pressure dependence of order parameters is analyzed, by discussing the role played by repulsive and attractive interactions between molecules.