Figure (a) Plot of the instanton pathways at a range of temperatures along with the MEPs connecting the reactant and product minima via the MECP. All paths are projected on the coordinate space spanned by the C-Cl-S-Cl dihedral angle and the difference between the two S-C-Cl bond angles. (b) Potential energy profile along the MEPs (dashed lines) and along the instanton at 175 K (solid lines). The triplet and singlet surfaces are colored in blue and orange respectively, and the MECP and instanton hopping point are highlighted by the black cross and diamond. The instanton energy, E I , at this particular temperature corresponds to the reaction-coordinate zeropoint energy at the triplet minimum, as depicted by the thin gray line. In order to best compare the path lengths, the instanton starts at the point at which the MEP crosses this line. inverted regime, according to the nomenclature borrowed from Marcus theory. Instead of relying on the MEPs, golden-rule instanton theory defines an optimal tunneling pathway, also called the "instanton". The tunneling pathway is formed of two paths, x e (u) and x g (u), one which travels on the excitedstate (triplet) PES, V e , and the other on the ground-state (singlet) PES, V g . Since here we aim to describe a deactivation process, the former corresponds to the reactant and the latter to the product state. The tunneling pathway is said to be optimal if it makes the Euclidean action