Structural, electronic, and optical properties of polyacetylene from a total-energy local-density molecular-cluster approach

A self-consistent local-density total-energy molecular-cluster approach is used to study several fundamental structural, electronic and optical properties of trans- and cis-polyacetylene (PA). The use of extended basis sets in the linear combination of atomic orbitals (LCAO) representation and of accurate multipolar potential representations was found to be essential for obtaining accurate results. Chain clusters Cₙ{H}ₙ₊₂$ with increasing n=8, 12, 16, and 20 were investigated in order to assure convergence of physical quantities of interest. The ground-state density of states, energy gaps, and binding energies are obtained and compared with experimental results and results of other calculations. A gap of 1.68 eV is obtained for the trans case and 1.74 eV for cis. $trans----PA is found to be a lower-energy state than cis-PA (by 0.1--0.2 eV per ${C}₂H₂). Accompanying the Peierls transition from a metallic to semiconductor state, the dimerization energy is determined from optimized total-energy calculations on trans-PA to be about 0.02--0.03 eV/C₂{H}₂$.