Correlaction Effects on the Band Gap of Conducting Polymers

By applying the projection technique to the com putation ofexcitation energies, we study the correlation e(cid:11)ects on the band gap of conducting polym ers. In the presenceofan additionalelectron orhole,thecorrelation inducesa polarization cloud around theadditionalparticle,which form sa polaron.Fortheexcitation energy ofa polaron,thereisa com petition between a loss ofthecorrelation energy in theground state and a gain of polarization energy. For the H ubbard interaction, the loss of correlation energy isdom inantand correlationsincrease the band gap.H owever,for long-range interactions,the gain ofpolarization energy isdom inantand correlations decrease the band gap. Screening the long-range interaction suppresses the gain of the polarization energy so that correlations again increase the band gap. A sm all dim erization is always favorable to the correlation e(cid:11)ects. For trans -polyacetylene, we obtain the on-site repulsion U = 4 : 4 eV and the nearest-neighbor interaction V = 0 : 8 eV .The screening of (cid:25) electrons due to the polarizability of (cid:27) electrons is quite strong.