BOND LENGTH ALTERNATION AND ENERGY-GAP IN (CH)X - APPLICATION OF THE INTERMEDIATE EXCITON FORMALISM

Previous ab initio LCAO Hartree-Fock (HF) self-consistent field (SCF) band structure calculations predicted the alternating all trans (CH)x (polyacetylene or polyene) to be energetically more stable than the equidistant metallic chain, however, the gap of those calculations was too large (≈7 eV). The present discussion based on semi-empirical PPP hamiltonian explains how electronic correlation neglected in the HF theory reduces this gap to ≈4 eV, while the excitation energy for the creation of excitons is estimated to be below 2 eV. The formalism for this estimation is an intermediate-binding exciton theory with explicit use of numerical Wannier functions. It is concluded that the gap is not due to a spin-density-wave state of the equidistant chain, as expected by Ovchinnikov . © 1979.