EFFECT OF CORRELATION IN THE PEIERLS-HUBBARD MODEL - SMALL CLUSTER STUDIES

The effect of Coulomb interaction in the Su-Schrieffer-Heeger model fo r polyacetylene is studied by means of exact diagonalization of small clusters. We determine the amplitude of the ground-state dimerization very precisely to find that it has a maximum at U/t0 approximately 2 o r 3. This result is compared with the previous calculations. For the e xcitation spectrum it is shown that the gap for the lowest singlet exc itation is roughly twice larger than that for the triplet excitation. This is interpreted in terms of a two-spinon bound state. The results in the large-U region are examined in the spin-Peierls Hamiltonian.