The original model developed by Hahn and Stock is a a two-electronic-state two-dimensional model for the photo-induced 11-cis to all-trans isomerization of retinal in rhodopsin. The effective nuclear modes 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" . Electronic ground (S 0 ) and first-excited (S 1 ) singlet states present a conical intersection (CI) at ϕ CI ± π 2 , q CI = 0, and are represented in Fig. (upper panel). These BO PESs are obtained by diagonalization of the electronic Hamiltonian Ĥel (ϕ, q) given in the diabatic basis