It is fortuitous that CTST with standard scale factors performs well on the KIE. As seen from the rate constant results, CTST with standard scale factors underestimates the rate constants for both CH 4 and CD 4 , and there are several sources of errors such as the missing variational optimization of the transition state location, the omission of a tunneling transmission coefficient, and the error in the zero-point energy due to using standard scale factors. These errors happen to accumulate similarly enough for CH 4 and CD 4 that the KIE is reasonably accurate. This error cancellation is not guaranteed to happen for all systems, and ample previous work shows that we cannot rely on CTST for accurate predictions of KIEs. Although CNEO-TST may still face challenges in the low-temperature deep tunneling regime, it has high promise as an inexpensive and black-box method for applications to more complex chemical and biological systems at room temperature.