First-principles determination of possible geometries in cis-polyacetylene

Based on the local-density approximation of density-functional theory, we p erform an ab initio calculation to determine possible geometries of cis-pol yacetylene. While the topological stable cis-transoid structure is confirme d, 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 t he two kinds of nonequivalent bonds that causes the nonexistence of the tra ns-cisoid structure.