Singlet-triplet energy gap of multiresonant molecular systems: A double hybrid time-dependent density functional theory study

Singlet–triplet energy gaps (ΔEST) for multiresonant molecular systems are investigated within time-dependent density functional theory (TDDFT), using conventional linear response TDDFT (LR-TDDFT) and double hybrid TDDFT (DH-TDDFT) schemes. LR-TDDFT methods systematically overestimate ΔEST; on the other hand, by explicitly taking double excitation characters into consideration, DH-TDDFT considerably improves the gap problem. Further theoretical analysis shows that DH-TDDFT calculations also work for the description of negative gap situations, which LR-TDDFT intrinsically fails. The present findings demonstrate the applicability of TDDFT-based methods for predicting ΔEST as a relatively low-cost computational approach, encouraging us to develop new exchange-correlation functionals suited for DH-TDDFT calculations.