The instanton rate thereby proves superior to the previously applied methods, due to its ability to treat heavy-atom tunneling in full dimensionality and without making a global harmonic and the results obtained with the Zhu-Nakamura formula (ZN) and the weak-coupling approximation (WC) in Ref. 45. The lines connecting the WC and ZN rates serve as a guide to the eye. (b) Instanton rates for a wider range of energies for CSCl 2 and 13 CSCl 2 . The kinetic isotope effect computed as the ratio between the two rates is shown in the inset. The energy is given relative to the zero-point energy of the reactant minimum.