Multidimensional Potential Energy Surfaces Resolved at the RASPT2 Level for Accurate Photoinduced Isomerization Dynamics of Azobenzene.

We have used state-of-the-art ab initio restricted active RASPT2 computations using a 16 orbitals, 18 electrons active space to produce an extended three-dimensional map of the potential energy surfaces (PESs) of the ground and first nπ* excited states of azobenzene along CNNC torsion and the two CNN bending angles, which are the most relevant coordinates for the trans-cis photoisomerization process. Through comparison with fully unconstrained optimizations performed at the same level of theory, we show that the three selected coordinates suffice to correctly describe the photoisomerization mechanism and the S1-S0 crossing seam. We also provide a map of the nonadiabatic coupling between the two states in the region where they get closer in energy. Eventually, we show that treating the two CNN bending angles as independent coordinates is fundamental to break the symmetry and couple the two electronic states. The accuracy of the S0 and S1 PESs and couplings was validated with semiclassical dynamics simulations in the reduced space of the scanned coordinates, showing results in good agreement with published full-coordinate dynamics.