Limitations of MRSF-TDDFT for Applications in Photochemistry

Mixed-reference spin-flip time-dependent density functional theory (MRSF-TDDFT) has recently emerged as an attractive electronic-structure method for studying photochemical processes, given that it bridges the computational efficiency of single-reference approaches with the versatility of multireference methods. In the following, we critically assess the general applicability of MRSF-TDDFT to photochemistry and identify two important limitations. First, the doubly excited configurations included in MRSF-TDDFT come at the cost of missing some singly excited configurations. Second, MRSF-TDDFT provides unreliable excited-state energies when its triplet reference─a cornerstone of the method─abruptly changes its nature, e.g., when the T1 and T2 triplet states become nearly degenerate and exchange electronic character. This change of character of the triplet reference can induce discontinuities or sharp distortions in electronic potential energy curves of the response states in unsuspected regions of the nuclear configuration space. We propose strategies and diagnostics to detect these limitations in the exploration of potential energy surfaces and nonadiabatic molecular dynamics using MRSF-TDDFT.