Density-functional crystal orbital study on the structures and energetics of polyacetylene isomers

Total energies and optimized molecular structures of the trans-transoid (Tt) and cis-transoid (Ct) forms of polyacetylene are calculated by the density-functional crystal orbital method. The Slater-Vosko-Wilk-Nusair (SVWN), the Becke-Lee-Yang-Parr (BLYP), and the Becke3-Lee-Yang-Parr (B3LYP) functionals are used with the (Formula presented) and (Formula presented) basis sets. Potential-energy curves of the Ct form along the bond-alternation coordinate [which represents the transition from the Ct form to the trans-cisoid (Tc) form] are calculated with the SVWN, BLYP, and B3LYP functionals. The SVWN and BLYP functionals seriously underestimate the double-minimum character of the potential-energy curves, so that the calculated potential-energy curves have no local minimum at the Tc structure. The potential-energy curves calculated with the B3LYP functional have distinct shoulders at the Tc structure, and the structural parameters of the Tc form are optimized with this functional. The structural parameters and ultraviolet photoelectron spectra of the Tt and Ct forms calculated by using the B3LYP functional are in reasonable agreement with the experimental results. Potential-energy curves along the CC-CC dihedral angle coordinate are calculated with the B3LYP functional. It is found that the calculated potential-energy curve has a shallow local minimum at the cis-gauche (Cg) form. The B3LYP functional predicts the total energies of the polyacetylene isomers increase in the order (Formula presented).