In this work we apply the newly developed golden-rule instanton theory for the inverted regime to the T 1 → S 0 spin crossover of thiophosgene based on on-the-fly multireference perturbation theory electronic-structure calculations and thereby show how the method can simulate spin-crossing reactions with unprecedented accuracy. We find that our treatment of multidimensional tunneling is necessary for a correct description of the reaction mechanism, which explains the significant improvement of our results compared with those of previous studies.