Full geometry optimizations on oligoenes have been performed with Hartree-Fock and density functional theory in combination with double zeta and triple zeta quality basis sets with primary focus on the degree of bond length alternation and on the energy gap. Monitoring the dependence of the computed properties on the oligomer size provides new insights into the reliability of the calculations, which are analyzed in terms of dynamical and nondynamical electron correlation. Our theoretical bond length alternation values for the oligomers of polyacetylene extrapolate to significantly smaller values than what has been established by experiments and earlier theoretical predictions. The exact exchange mixing to the exchange-correlation functional not only improves the agreement of the theoretical gap of oligoenes with experimental excitation energies but also increases the computed bond length alternations. Based on a newly proposed one parameter functional of Becke, the effect of the exact exchange mixing has been further identified, and a practical measure of nondynamical correlation energy has been suggested. The popular Becke-Lee-Yang-Parr density functional produces bond alternation values that are unacceptably small. (C) 1997 American Institute of Physics. [S0021-9606(97)52441-X].