CASPT2//CASSCF and TDDFT//CASSCF Mapping of the Excited State Isomerization Path of a Minimal Model of the Retinal Chromophore

In this work we explore the validity of the application of TDDFT methods to the study of excited state reactivity problems. Accordingly, TDDFT//CASSCF calculations have been used to evaluate the excited state isomerization path of a retinal chromophore model and have been compared with the path obtained at the more expensive CASPT2//CASSCF level. We show that the TDDFT and CASPT2 excited state energy profiles are qualitatively similar. Indeed, remarkably, the TDDFT//CASSCF strategy achieves a qualitatively correct description of the intersection region, which is a basic mechanistic feature of photochemical processes. Quantitative differences are found in the region of the energy profile characterized by a coupled stretching-twisting deformation. This discrepancy reflects the difference in the equilibrium values of the bond lengths of the planar excited state structures when evaluated at the TDDFT and CASPT2 levels. We stress that our results support the use of TDDFT for the evaluation of energy profiles along CASSCF reaction coordinates. Thus, in no way shall such results be considered as indicative of the validity of TDDFT for the calculation of excited state equilibrium structures or reaction coordinates.