Previous theoretical studies assumed that tunneling followed a predetermined reaction coordinate. We can now confirm that this was the cause of the order-of-magnitude deviation between their predictions and the experimental measurements. Further approximations of the potential along the reaction coordinate prevent these one-dimensional methods from even capturing the qualitative low-temperature behavior. In some cases, there may be a huge error cancellation from the two approximations, one which broadens and one which narrows the barrier. This explains the widely varying quality of results obtained with NA-TST/WC that can be found in the literature. Instanton theory avoids these issues by locating the optimal reaction mechanism at each temperature/energy of interest in a fully ab initio manner. This has allowed us to unravel the substantial role of multidimensional quantum nuclear tunneling in this reaction, which proceeds by a corner-cutting pathway. This study thus emphasizes the crucial role of heavyatom tunneling in nonadiabatic reactions which speeds up rates by several orders of magnitude, especially in the inverted regime where the crossing potentials form a narrow cusp. In particular we predict a measurable kinetic isotope effect between CSCl 2 and 13 CSCl 2 , which we hope to be confirmed by future experiments.