GROUND-STATE AND EXCITATIONS IN POLYACETYLENE CHAINS

While a nearest-neighbor-only Su-Schrieffer-Heeger model cannot distin guish cis- from trans-polyacetylene, a simple extension which includes a third-neighbor interaction can. This extended model gives a nondege nerate cis structure with a larger band gap and a smaller cohesive ene rgy 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 confor mational excitations in both isomers are found to be repelled from the chain ends.