An incremental approach for correlation contributions to the structural and cohesive properties of polymers. Coupled-cluster study of trans-polyacetylene Original Research Article

An ab initio approach is used to study the structural and cohesive properties of the electronic ground state of trans-polyacetylene. Starting from the Hartree-Fock approximation the correlation contributions of the valence electrons were evaluated by means of increments obtained for localized bonds and for pairs and triples of such bonds. Individual increments were determined using the coupled-cluster approach with single and double excitations augmented by a fourth-order perturbative estimate of the contribution of connected triples. The valence correlation contribution to the cohesive energy at the polarized valence-triple-zeta basis set level is 2.02 eV per CH unit (21%) for the equilibrium structure. The dimetrization parameter as a measure of bond alternation is overestimated at the Hartree-Fock level by about 50%, whereas the inclusion of electron correlation at the coupled-cluster level yields a value of 0.028 Å in excellent agreement with the experimental result of 0.026 Å. Additional data is given for the less stable forms cis-transoid- and trans-cisoid-polyacetylene.