Subject of investigation is the primary event of vision characterized by photo-isomerization of the retinal chromophore in rhodopsin. Ground and excited state ab initio molecular dynamics of different four double bond chromophore models were performed to study the effect of molecular distortion and influence of the methyl groups. Initial geometries and velocities of the trajectories were calculated by zero-point energy sampling. After excitation both electronical states were followed simultaneously by state-averaged CASSCF, including all 8 π-orbitals and -electrons in the active space. To determine the nonadiabtic transition two methods were applied: one is based on the change of the configuration vectors in two succeeding steps, while the other is an analysis of the torsional modes. The results show the distortion of the chromophore to be crucial for the selectivity of the double bond and the increase of the product yield. A methyl group in α position to the cis double bond was found to reduce the reaction time.