Ground state and excitations in polyacetylene chains

While a nearest-neighbor-only Su-Schrieffer-Heeger model cannot distinguish {ital cis}- from {ital trans}-polyacetylene, a simple extension which includes a third-neighbor interaction can. This extended model gives a nondegenerate {ital cis} structure with a larger band gap and a smaller cohesive energy than {ital trans}, and yields the correct order of the total energies of the {ital trans}, {ital cis}-{ital transoid}, and {ital trans}-{ital cisoid} structures. It also produces bound bipolarons or excitons on doubly charged {ital cis} chains. All conformational excitations in both isomers are found to be repelled from the chain ends. {copyright} {ital 1996 The American Physical Society.}