Binding energy of holes in the two chains

I have computed the binding energy, E-B, of two holes at 1/4 filling in ter ms of electrons in the two chains of organic compounds within an Su-Schrief fer-Heeger Hubbard model with the intrachain second- and third-neighbor-tra nsfer energies. I have used an adiabatic approximation, a mean-field approx imation and periodic boundary conditions. As the electron-lattice interacti on, lambda, increases, E-B is negative and decreases. On the other hand, as the on-site Coulomb repulsion, U, increases, E-B 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 E-B, is negative. It turns out tha t A and U play an important role in the electronic states of the two chains of organic compounds.