Exciton binding energy in the strong correlation limit of conjugated chains

By applying the numerically accurate symmetrized density-matrix renormaliza tion-group method coupled with the extended Hubbard-Peierls model, we find that (i) the on-site Hubbard repulsion energy U dramatically reduces the bi nding energy of the lowest optically allowed 1B(u) exciton; (ii) in the zer o-dimerization limit, there exists a critical value of V at which the 1B(u) exciton becomes bound; the critical value V-c = 2t is fully in agreement w ith the recent analytical results at the infinite-ii limit by Gallagher and Mazumdar [Phys. Rev. B 56, 15025 (1997)]. Furthermore, this critical value decreases appreciably for weaker on-site correlation strengths, when the d imerization amplitude (delta) is nonzero; The present accurate numerical re sults contradict those obtained recently by Yu, Saxena, and Bishop [Phys. R ev. B 56, 3697 (1997)] both qualitatively and quantitatively. We also prese nt first-order perturbation plus random-phase-approximation and single conf iguration-interaction analyses to rationalize the numerical calculations. [ S0163-1829(98)02244-9].