In deriving more elaborate approximations to the dispersion energy in the Eq. ( ) it might be essential to use a computer-aided second-quantized-algebra system that enables automatic derivation and implementation of orbital formulas. For a single Fermi vacuum, such implementations are already known. Williams et al. implemented a similar system for the product Fermi vacuum ; 15 recently, however, we have implemented a more convenient system in our group, and our future work will include the development of more accurate schemes. Obviously, highly-correlated dispersion-free interaction energy is difficult to obtain in the SAPT theory. However, composite schemes, which use preprocessed supermolecular interaction energies combined with perturbation theory corrections, can work very efficiently.