The stretching mode q is also called coupling mode, as the potential coupling V 10 only depends on q. 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. The adiabatic potential energy surfaces (PESs) S 0 and S 1 are obtained by diagonalization of the electronic Hamiltonian in Eq. . The PESs present conical intersections (CIs) at ϕ CI ± π 2 , q CI = 0. The cis conformer is characterized by the value of the reactive coordinate ϕ cis = 0, and at this geometry for q = 0 the excitation energy from S 0 to S 1 lies in the visible domain. The adiabatic PESs are shown in Fig. as colormaps, where the positions of the CIs are indicated with crosses. At the cis geometry and for slightly negative values of the coupling mode, S 0 is a potential well while S 1 has a saddle shape; at the trans geometry (ϕ = ±π), only S 0 presents stable configurations.