The photochemical cis-trans isomerization of a five double bond 11-cis retinal model has been investigated using ab initio molecular dynamics in vacuo and zero-point energy sampling. The trajectories show that the reaction in the excited-state is restricted to the CIO-C13 fragment of retinal. The protein-induced torsion of the C11=C12 and C12-C13 bonds are sufficient to maintain the stereoselectivity and the quantum efficiency of the retinal photoconversion in rhodopsin.