3.1. Barrier Heights. Figure examines errors for barrier heights, using the functionals described in Section 2 .2. In Fig. , these errors are separated according to the various data sets that comprise the barrier height data in GMTKN55. The BHROT27 data set, consisting of 27 different rotational barriers around a single bond, is clearly an outlier insofar as the errors are very small for every functional examined. Although rotational barriers can manifest SIE problems in molecules with extended conjugation, 153 these particular molecules do not fall into that category. The transition states in question do not involve incipient bonds or fractional charges, and the er- rors are insensitive to the fraction of exact exchange.