To obtain practical, working equations for the cluster expansion of the dispersion energy, we should expand the Eqs. ( ) and ( ) using the Baker-Hausdorff-Campbell formula and examine which terms (in terms of the power of W operator) of such expansions are important. This information is crucial for introducing any approximate scheme: the proper working approximation should be exact to the second order of intramonomer correlation since the numerous tests have shown that the perturbation theory with λ A + λ B = 2 [cf. Eq.