Abstract
<jats:p>Photochemical electrocyclic reactions play a fundamental role in photoswitching and biological processes, yet they pose severe challenges for electronic structure methods. Here, we benchmark the cost-effective mixed-reference spin-flip time-dependent density functional theory (MRSF-TDDFT) method for capturing the photoinduced electrocyclic interconversions between 1,3-cyclohexadiene and cis-1,3,5-hexatriene, using the extended multi-state complete active-space second-order perturbation theory (XMS-CASPT2) as the reference method. We show that MRSF-TDDFT predicts photorelaxation potential energy profiles that closely match those of XMS-CASPT2, accurately capturing both the change in the character of the lowest excited state and its minimum-energy conical intersection with the ground state. Moreover, surface hopping nonadiabatic molecular dynamics simulations indicate that MRSF-TDDFT not only predicts excited-state lifetimes and photoisomerization quantum yields in close agreement with XMS-CASPT2, but also explores a remarkably similar extended conical intersection seam with the ground state. Specifically, both the MRSF-TDDFT and XMS-CASPT2 dynamics reveal that the ring-opening and ring-closing pathways involve two distinct regions of this intersection seam. Ultimately, this work demonstrates that MRSF-TDDFT can accurately describe photochemical electrocyclic reactions, establishing a solid foundation for simulating these processes in larger molecular systems.</jats:p>