For instance, the 1 ππ * ↔ 3 ππ * and 1 nπ * ↔ 3 nπ * conversions are forbidden due to the lack of orientational changes of spatial orbitals. As implied above, therefore, same-configuration ISCs must involve the cooperation of IC and ISC, e.g., ππ * IC -→ 3 nπ * ISC --→ 1 ππ * , emphasizing the importance of treating IC and ISC on the equal footing in many dynamical processes involving spin-flipping. In addition, recent experimental advances of X-ray spectroscopy also require accurate computational methods with the proper spin-orbit couplings (SOC) to account for the spectra of heavy element compounds. The ISC plays a major role in the phenomenon of thermally activated delayed fluorescence (TADF) as well, in which the triplet-to-singlet ISC facilitates the harvesting of the triplet excitons in organic light-emitting diodes (OLEDs). As the typical system size of TADF materials increases, efficient quantum chemistry methods are highly desirable for the computations of SOCs among various excited states.