The Franck-Condon structure of the lowest lying intense polyenic electronic transition of trans, trans-, cis, trans-, and cis, cis-octatetraene is investigated through model calculations. To avoid a possible bias in the parameters of the model, the starting inputs are obtained ab initio. The molecular orbital (MO) procedure consists in first optimizing the structures of S-0 and S-2 and then calculating the vibrational frequencies at the stationary points on the potential energy surface of the three isomers. Together with the minima associated with the three isomers, we find one more saddle point in S-0 and two more in S-2. These three saddle points correspond to planar S-0 cis, cis-octatetraene, planar S-2 cis, cis-octatetraene, and planar S-2 cis, trans-octatetraene. The displacement, between the surfaces, of the harmonic oscillators associated with the normal modes, are obtained and used to simulate the Franck-Condon activity of the S-0 --> S-2 transitions of the three isomers. Such displacements are calculated in two schemes, the first uses the variation of the equilibrium position of the vibrational oscillators in the two states involved in the transition and requires full geometry optimization of all the geometrical parameters of the two states; the second, approximate, scheme requires only a single point calculation on the excited state surface and is therefore far less demanding. A simple scaling procedure, proposed before for hexatriene, is used to improve the agreement between theory and experiment. The model calculations of the Franck-Condon structure simulate very well the S-0 --> S-2 absorption spectra of trans, trans- and cis, trans-octatetraene. It is further proposed that the large homogeneous linewidth in the S-0 --> S-2 transitions of polyenic systems is a function of the nonplanarity of these molecules in S-2.