Modeling Xanthophyll Excited States via Cost-Effective Quantum Chemistry methods and Property-Based Diabatization

Xanthophyll carotenoids play essential roles in photosynthetic light harvesting and photoprotection in biological systems, yet the accurate description of their excited states at a feasible computational cost remains challenging due to their extended π-conjugation and multireference character. Here, we compare the DFT/MRCI, FOMO-CI, and mixed-reference spin-flip TDDFT (MRSF-TDDFT) descriptions of the excited states for a representative set of xanthophylls. We analyze vertical excitation energies and potential energy profiles along the bond-length alternation coordinate, adopting a multiple-property-based diabatization (MPD) scheme to consistently characterize electronic states across methods. We find a remarkably consistent description of the low-lying excited states and their potential energy curves across the methods. DFT/MRCI and the semiempirical FOMO-CI exhibit closely aligned behavior across xanthophylls, while the performance of MRSF-TDDFT depends on the choice of exchange-correlation functional. Overall, our calculations suggest that the covalent 2A_g^- state lies below the ionic 1B_u⁺ at the Franck-Condon point. Our analysis provides general insight into the electronic structure of xanthophylls, including the interplay between covalent and ionic configurations induced by structural distortions and environmental effects. The MPD framework enables a direct comparison of state character, highlighting the similarities between methods and clarifying the origin of remaining differences. This work establishes a consistent framework for understanding xanthophyll photophysics and offers practical guidance for modeling their excited states with cost-effective methods.