WOS:000288410100035 ; International audience ; Rhodopsin (Rh) and bathorhodopsin (bathoRh) quantum-mechanics/molecular-mechanics models based on ab initio multiconfigurational wave functions are employed to look at the light induced pi-bond breaking and reconstitution occurring during the Rh -\textgreater bathoRh and bathoRh -\textgreater Rh isomerizations. More specifically, semiclassical trajectory computations are used to compare the excited (S-1) and ground (S-0) state dynamics characterizing the opposite steps of the Rh/bathoRh photochromic cycle during the first 200 fs following photoexcitation. We show that the information contained in these data provide an unprecedented insight into the sub-picosecond pi-bond reconstitution process which is at the basis of the reactivity of the protein embedded 11-cis and all-trans retinal chromophores. More specifically, the data point to the phase and amplitude of the skeletal bond length alternation stretching mode as the key factor switching the chromophore to a bonding state. It is also confirmed/found that the phase and amplitude of the hydrogen-out-of-plane mode controls the stereochemical outcome of the forward and reverse photoisomerizations.