The potential coupling V 10 only depends on q, thus the stretching mode is also called coupling mode. The parameters of the model are: the effective mass of the reaction coordinate m -1 = 4.84 • 10 -4 eV is chosen so as to achieve isomerization within 200 fs; the frequency of the coupling mode is ω = 0.19 eV and the interstate coupling is λ = 0.19 eV, with κ = 0.1 eV indicating a gradient in the excited state; the parameters W 0 = 3.6 eV, W 1 = 1.09 eV and E 1 = 2.48 eV are chosen in order to match the S 0 /S 1 gap to the center frequency of the absorption bands of the cis and trans isomers. At the cis geometry (ϕ cis = 0), the excitation energy ∆E ex to induce a S 0 -to-S 1 transition at q = 0 lies in the visible domain, being ∆E ex = 2.48 eV (λ ex = 500 nm). The model is parametrized to reproduce features observed in Raman and time-resolved experiments , and was for long time the preferred choice to study the ultrafast relaxation process of the photo-excited retinal based on quantum-dynamical simulations .