Correlation effects on bond alternation in polyacetylene

The influence of the electronic interactions on bond alternation in trans-polyacetylene is examined on the basis of the Pariser-Parr-Pople Hamiltonian. The ground state of the dimerized chain is studied within a recently developed local approach to the computation of electronic correlation energies. For a Hubbard-type interaction it is shown that electronic correlations are the principle driving mechanism for the dimerization. When the long-range part of the Coulomb interaction is included, the correlations turn out to stabilize the undimerized structure. A rather weak static screening due to $\sigma$ electrons, however, is sufficient to recover the instability found for the Hubbard interaction.