The kinetic isotope effect (KIE) for the CH 4 and CD 4 reactions with OH is presented in Figure . The experimental curve is almost linear on the Arrhenius plot. CTST with the standard scale factor performs exceptionally well, yielding the most accurate KIE predictions. The good agreement results from predicting similar underestimates for protium and deuterium transfer. CVT/SCT with the standard scale factor has a more physically correct picture but underestimates the KIE. With the SRP scale factor, the KIE is significantly increased and becomes overestimated, and only with the SRP scale factor transferred from CH 4 OH ‡ to CD 4 OH ‡ can the KIE be reasonably described. In contrast, without any scaling, CNEO-TST gives a prediction that is reasonably close to the experimental results although with a slight underestimation. This underestimation is because CNEO gives a slightly larger error for deuterium, which is heavier and thus behaves more classically than hydrogen.