LACK OF A TRANS-CISOID PHASE IN CIS-POLYACETYLENE

By an ab initio calculation based on the local-density approximation o f density-functional theory, the possible geometries of cis-polyacetyl ene are determined. While the stable cis-transoid structure is confirm ed, the ''metastable'' trans-cisoid phase is not found in our first-pr inciples calculation. Further calculations indicate that the electroni c band gap in cis-polyacetylene is contributed almost equally by the P eierls dimerization and the nonequivalence of its two kinds of bonds. This fact is shown to account for the nonexistence of the trans-cisoid phase in cis-polyacetylene.