Is MRSF-TDDFT Suitable for Cyclobutanone Dynamics? The Role of Higher Energy States in Surface Hopping Simulations

Modelling 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 multi-reference character, as well as an accurate description of the ground state topology due to 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 the inclusion of 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 emphasise the importance of carefully selecting the number of electronic states in the dynamical manifold.