Determination of the bond length alternation of polyacetylene, polydiacetylene and polybutatriene from MP2 oligomeric investigations

MP2/6-311G* full geometry optimizations have been carried out for increasingly large oligomers of polyacetylene (PA), polydiacetylene (PDA) and polybutatriene (PBT). Extrapolations to the polymeric limit have been performed. We propose new limits for the bond length alternation (BLA) of infinite PA. The inclusion of electron correlation effects modifies the BLA patterns of PDA and PBT and makes easier the classification of the substituted polydiacetylenes.