In this section, we introduce the following notation. The quantities with subscripts A and B refer to operators which act on A or B monomer wavefunctions, respectively. We introduce the general excitation operators τ with subscripts I, J and M, N referring to the excitations of Slater determinants describing monomers A and B, respectively. We assume that both A and B monomers are in non-degenerate ground states, and for both monomers, obtaining the reference Hartree-Fock states is possible. The second assumption we need is that for each monomer, we can solve coupled cluster equations at the same excitation level. In other words, we desire that for arbitrary excitation operators τ I , τ M acting on Slater determinants of A and B, respectively, we have following equations fulfilled: