Abstract The method of orbital interaction describing the intermolecular interaction is reformulated in terms of Green’s function. The eigen-values and eigen-vectors for individual molecules obtained by block diagonalization of the original Hamiltonian, are sufficient to predict the next step of the reaction. The polarizabilities in the ordinary perturbation treatment is also effectively used. This procedure is further clarified by defining the effective coupling. The method is applied to a molecular compound between ethylene and H2.