As shown in Fig. , the instanton rates are orders of magnitude faster than the fully classi- cal rates as well as NA-TST predictions, which quantize vibrations but neglect tunneling completely. In the low-temperature limit, instanton theory recovers the expected plateau of a unimolecular rate process. Since none of the other methods is able to capture the temperature-dependence of the tunneling mechanism, it comes as no surprise that they fail to describe the plateau correctly. For instance, we considered the Wentzel-Kramers-Brillouin (WKB) approximation to Fermi's golden rule. Although the WKB formula is able to qualitatively describe the fact that the low-temperature rate plateaus, it is orders of magnitude too small. This is because the MEPs are suboptimal pathways which are significantly different from the optimal instanton pathways at low temperatures. The effective barrier seen by WKB is thus much too broad, leading to the dramatic underestimation of the rate observed in Fig. . Both versions of the WC approach 45,55 make a further approximation to this by replacing the potentials by linear functions which cross at the MECP. Although this approximation is valid at high temperatures where the instanton is collapsed close to the MECP, it clearly cannot correctly describe the optimal tunneling pathway as it approaches the reactant minimum. In particular, the linear approximation makes the effective barrier narrower than that used by WKB, which fortuitously corrects some of the errors and explains why some of the WC(min) rates in particular are in better agreement with instanton theory than WKB. Nonetheless, the mechanism is fundamentally wrong and does not lead to a low-temperature plateau. This analysis illustrates the importance of treating nuclear tunneling as a multidimensional phenomenon for both quantitative and qualitative arguments.