The SOC was calculated at the hopping point for each instanton on the same level of electronic-structure theory as for the other calculations. In each case, it was found to lie between 143 cm -1 and 194 cm -1 and has a value of 156 cm -1 at the MECP, which is practically identical to the results of previous studies. Note that the slight kink in the thermal instanton rates (Fig. ) arises from an abrupt change of the SOC as the mechanism changes near 200 K. Including the SOC explicitly in the instanton optimization would likely smooth out this unphysical kink, since the path would then find a compromise between finding the optimal action and the largest possible SOC. This is however unnecessary here as the effect is minor.