Photoisomerization of Model Retinal Chromophores: Insight from Quantum Monte Carlo and Multiconfigurational Perturbation Theory

We present a systematic investigation of the structural relaxation in the excited state of model retinal chromophores in the gas phase using the complete-active-space self-consistent theory (CASSCF), multiconfigurational second-order perturbation theory (CASPT2), quantum Monte Carlo (QMC), and coupled cluster (CC) methods. In contrast to the CASSCF photoisomerization mechanism of bond inversion followed by torsion around formal double bonds, we find that the other approaches predict an initial skeletal relaxation which does not lead to bond inversion but to a rather flexible retinal chromophore with longer bonds and with the bond-length pattern of the ground state being partly preserved. The relaxation proceeds then preferentially via partial torsion around formal single bonds and does not reach a conical intersection region. Our findings are compatible with solution experiments which point to the existence of multiple minima and relaxation pathways, some of which are nonreactive, do not lead to photoproducts via conical intersection, and are dominant in solution. Our results also demonstrate the importance of a balanced description of dynamical and static correlation in the excited-state gradients and raise serious concerns on the common use of the CASSCF method to investigate structural properties of photoexcited retinal systems.