With the standard scale factor 0.973, CTST with M08-HX significantly underestimates the reaction rate constants at and below room temperature. The CVT rate constant is even lower. However, CVT/SCT leads to great improvement. These performances are similar to those observed in the D + H 2 --→ DH + H case. The underlying reason remains that the CVT optimization decreases the predicted rate constants by identifying the highest free energy barrier along the MEP, while the tunneling contribution increases the rate constants.