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 M oller-Plesset (MP2) levels are performed for finite polyenes C2nH2n+2 to estimate the structure and dimerization energy (E-dim) of polyacety lene. The effect of electron correlation on the structure of finite po lyenes is analyzed in detail. The MP3/6-31G C=C and C-C bond lengths in polyacetylene are estimated by a simple extrapolation method using empirical corrections for the MP2 deficiencies, yielding values [C=C(M P3) similar to 1.36 Angstrom and C-C(MP3) similar to 1.44 Angstrom] th at are in a good agreement with experiment (C=C=1.36 Angstrom and C-C= 1.44-1.45 Angstrom). Comparison is also made with other theoretical es timates of polyacetylene structure. E-dim is approximated by the energ y difference between the equilibrium and hypothetical polyenic structu res. It is estimated that E-dim is similar to 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 b asis sets and similar to 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 se nsitive to the level of approximation employed so that a proper treatm ent of electron correlation is essential to obtain a reliable estimate of the dimerization energy. (C) 1997 John Wiley & Sons, Inc.