Is MRSF-TDDFT suitable for cyclobutanone dynamics? The role of higher energy states in surface hopping simulations

Modeling photochemical reactions remains a significant challenge due to the need for accurate descriptions of multiple excited states and their couplings with nuclear vibrations. Despite considerable advances in the field, the predictive power of current methodologies is still not fully established. Motivated by the recent Journal of Chemical Physics prediction challenge on cyclobutanone photochemistry, we simulated the photochemistry of the excitation of cyclobutanone by a 200 nm laser pulse using decoherence-corrected fewest-switches surface hopping, with a specific focus on the electronic structure method: mixed-reference spin-flip (MRSF) TDDFT. This promising method allows the description of S1-S0 conical intersections due to the ground state being a response of two triplet reference states, without the problem of spin contamination associated with regular spin-flip TDDFT. The simulated results show several decay pathways, most commonly through a ring opening S2/S1 conical intersection, resulting in the various photoproducts of carbon monoxide with some variation of C3 species, as well as the formation of ethene and ketene. Our simulations indicate that including higher-lying electronic states is essential to capture the diabatic trapping of the initial Rydberg 3s character, facilitated by an S3/S2 conical intersection. We emphasize the importance of carefully selecting the number of electronic states in the dynamical manifold. Comparison with XMS-PT2 confirms the presence of an S3/S2 conical intersection, although with a different topology and geometry closer to the Franck-Condon region. We also highlight the sensitivity of MRSF-TDDFT results to the choice of functional and the need for further validation of its conical intersection topologies to extend the method's applicability in nonadiabatic dynamics.