Having discussed the relation of the dispersion energy formula derived in Section 2.1 to the MBPT series expansion given in the literature, we can now ask the question of how the effects of intramonomer correlation can be introduced into the dispersion energy in a nonperturbative manner, using the converged cluster amplitudes and σ operator. As we have stressed in previous sections, a satisfactory simplification of the Eqs. Eqs. ( ) and (18) must remain exact to the second order of intramonomer correlation. First, note that the S operator can be simplified to T = T 1 +T 2 only since higher order terms in T do not contribute to the second-and lower orders in the intramonomer correlation. The connected triples (T 3 ) do not contribute to the second-order dispersion either. One can propose the simplest and most efficient cost approximation in a two-step procedure. First, we restrict the equations for the operator σ in Eq. ( ) to σ 11 only (we omit σ 21 and σ 12 ) and appropriate powers of T in the nested commutator expansion resulting from the e -T A H A e T A , e -T B H B e T B series to get the following equation: