There exist multiple spectroscopic methods that allow for time-resolved measurements of spin-crossing reactions. Due to its extraordinary properties, thiophosgene (CSCl 2 ) has been the object of several of those studies and is considered a prime example for studying fundamental concepts of spin crossovers. Thiophosgene is one of very few, if not the only known molecule which exhibits a shorter lifetime (∼20 ns) for the lowest triplet state, T 1 , than for the first excited singlet state, which creates ideal conditions for accurately probing the T 1 → S 0 decay. This is due to the fact that the internal conversion between the singlet states is so inefficient that both the S 1 and the S 2 state decay almost exclusively via fluorescence, thus breaking Kasha's rule. There have been several attempts for over 20 years to develop a quantitative theory for the spin crossover of thiophosgene. The development of accurate nonadiabatic rate theories is an active field of research itself and various approaches have been tried.