First-principles determination of possible geometries in cis-polyacetylene

Based on the local-density approximation of density-functional theory, we perform an ab initio calculation to determine possible geometries of cis-polyacetylene. While the topological stable cis-transoid structure is confirmed, the so-called “metastable” trans-cisoid phase is not found in this first-principles calculation. It is shown that it is the large band gap due to the two kinds of nonequivalent bonds that causes the nonexistence of the trans-cisoid structure.