Ab initio mol. dynamics (MD) calcns. have been performed to study the photoisomerization of a 3-double-bond retinal model chromophore, the all-trans-4,6-dimethylpenta-3,5-dieniminium cation, and the possible influence of non-planar distortions on the product distribution. In total, 171 trajectories have been generated for four different conformations of the structure, a planar one and three in which the C(4)-C(5) and the C(5)=C(6) bonds were increasingly twisted out of plane. Starting geometries randomly distributed about the equil. geometry were generated by zero-point energy sampling; trajectories were calcd. using CASSCF-BOMD methodol. and were followed until the photoproduct and its configuration could be assigned. For the latter, two different approaches were applied, one involving the CASSCF configuration vectors, the other an anal. of the MD at the first possible hopping event. Isomerization was found to occur almost exclusively about the central C(3)=C(4) double bond in the case of the planar model compd. Twisting the conjugated p-system shifts the isomerization site from the central double bond to the terminal C(5)=C(6) double bond. With both the C(4)-C(5) and the C(5)=C(6) bonds twisted by 20 Deg, about 35% of the trajectories lead to the configurationally inverted 5-cis product. The results are discussed with ref. to the highly selective and efficient photo-induced isomerization of the retinal chromophore in rhodopsin.