Estimates of the structure and dimerization energy of polyacetylene from ab initio calculations on finite polyenes

Ab initio calculations at the Hartree‐Fock (HF) and the second‐order Møller‐Plesset (MP2) levels are performed for finite polyenes C 2 n H 2 n +2 to estimate the structure and dimerization energy ( E dim ) of polyacetylene. The effect of electron correlation on the structure of finite polyenes is analyzed in detail. The MP3/6–31G* C(DOUBLE BOND)C and C(SINGLE BOND)C bond lengths in polyacetylene are estimated by a simple extrapolation method using empirical corrections for the MP2 deficiencies, yielding values [C(DOUBLE BOND)C(MP3) ∼ 1.36 Å and C(SINGLE BOND)C(MP3) ∼ 1.44 Å] that are in a good agreement with experiment (C(DOUBLE BOND)C (DOUBLE BOND) 1.36 Å and C(SINGLE BOND)C (DOUBLE BOND) 1.44–1.45 Å). Comparison is also made with other theoretical estimates of polyacetylene structure. E dim is approximated by the energy difference between the equilibrium and hypothetical polyenic structures. It is estimated that E dim is ∼ 1.4–1.5 kcal/mol (0.06–0.07 eV) per carbon‐carbon bond at the HF level with 4–21G and 6–31G* basis sets and ∼ 0.3–0.5 kcal/mol (0.013–0.022 eV) at the MP2 level with the 6–31G* basis set. It is concluded that E dim is very sensitive to the level of approximation employed so that a proper treatment of electron correlation is essential to obtain a reliable estimate of the dimerization energy. © 1997 John Wiley & Sons, Inc.