This paper describes an ab initio method for estimating intermolecular potential parameters required for equations of state. The intermolecular potentials are thermodynamically averaged over the Euler angles of the system as proposed by Ree to give a spherical average at that intermolecular distance. An alternative treatment using the multipole moments of the angular dependent potentials is also described. The results obtained in a detonation state calculation for the explosive FEFO agree well with those measured in experiments. ©2000 American Institute of Physics.