The model Hamiltonian used in this work allows us to describe the 11-cis to all-trans isomerization of the retinal chromophore in rhodopsin . Photoabsorption takes the cis conformer from the electronic ground state S 0 to the first excited state S 1 , thus initiating an isomerization reaction towards the trans conformer. The nuclear coordinates defining the model are an angular reaction coordinate ϕ and a collective vibration q characterizing "a delocalized stretching motion of the polyene chain, whereby single and double bonds interchange" .