Unveiling controlling factors of the S0/S1 minimum-energy conical intersection (3): Frozen orbital analysis based on the spin-flip theory

Abstract

Conical intersections (CIs), which indicate the crossing of two or more adiabatic electronic states, are crucial in the mechanisms of photophysical, photochemical, and photobiological processes. Although various geometries and energy levels have been reported using quantum chemical calculations, the systematic interpretation of the minimum energy CI (MECI) geometries is unclear. A previous study [Nakai et al., J. Phys. Chem. A 122, 8905 (2018)] performed frozen orbital analysis (FZOA) based on time-dependent density functional theory (TDDFT) at the MECI formed between the ground and first electronic excited states (S0/S1 MECI), thereby inductively clarifying two controlling factors. However, one of the factors that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy gap became close to the HOMO–LUMO Coulomb integral was not valid in the case of spin-flip TDDFT (SF-TDDFT), which is frequently used as a means of the geometry optimization of MECI [Inamori et al., J. Chem. Phys. 152, 144108 (2020)]. This study revisited the controlling factors using FZOA for the SF-TDDFT method. Based on spin-adopted configurations within a minimum active space, the S0–S1 excitation energy is approximately represented by the HOMO and LUMO energy gap ΔεHL, a contribution from Coulomb integrals JHL″ and that from the HOMO–LUMO exchange integral KHL″. Furthermore, numerical applications of the revised formula at the SF-TDDFT method confirmed the control factors of S0/S1 MECI.