Binding energy of holes in the two chains

I have computed the binding energy, EB of two holes at 1/4 filling in terms of electrons in the two chains of organic compounds within an Su-Schrieffer-Heeger Hubbard model with the intrachain second- and third-neighbor-transfer energies, I have used an adiabatic approximation, a mean-field approximation and periodic boundary conditions. As the electron-lattice interaction, λ, increases, EB is negative and decreases. On the other hand, as the on-site Coulomb repulsion, U, increases, EB is negative and increases. Thus two holes form a bound state at 1/4 filling in terms of electrons in the two chains of organic compounds, as EB is negative. It turns out that λ and U play an important role in the electronic states of the two chains of organic compounds.