Before tackling the rate problem, we first require an accurate description of the PESs of thiophosgene. As has been pointed out in Ref. 45, a multiconfigurational treatment is crucial since the electronic wavefunction at the MECP exhibits considerable contributions from several Slater determinants. Moreover, for quantitative comparisons to experiment it is important to not only include static but also dynamical correlation. We therefore employed single-state multireference Møller-Plesset perturbation theory (MRMP) throughout.