Accurate Prediction of Inverted Singlet-Triplet Excited States Using Self-Consistent Spin-Opposite Perturbation Theory.

The violation of Hund's rule, resulting in an inverted singlet-triplet (INVEST) gap, opens a new door in photophysics with major implications for OLED technology. However, accurately predicting negative singlet-triplet energy gaps typically demands prohibitive computational costs. In this study, we evaluate the efficacy of our recently developed one-body Møller-Plesset perturbation theory (OBMP2) and its spin-opposite variant (O2BMP2) as efficient alternatives. Benchmarking against 30 INVEST molecules reveals that O2BMP2, with appropriate spin-opposite scaling, achieves the accuracy of higher-level methods. Furthermore, with a formal scaling of O(N5) that is reducible to O(N4) upon implementation of density fitting approximations, O2BMP2 offers a good compromise of accuracy and computational efficiency for large-scale INVEST screening.