Ground state and excitations in polyacetylene chains

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