In the remainder of this work, reference geometries ('exact' G 0 ones entering Eq. 2) are taken from the B2se dataset of Barone and coworkers. This dataset contains accurate equi-librium structures of 68 molecules that have been obtained from the SE approach. Within the SE approach, equilibrium structures are derived from experimental ground-state rotational constants in which vibrational contributions computed by a suitable quantum mechanical (QM) methods are subtracted. Thereby, the SE structures match our definition of G 0 , as they correspond to the minima on the Born-Oppenheimer potential energy surface. In terms of applicability and accuracy, the SE approach offers a range of advantages for accurate structure determinations over only experimental or only theoretical approaches (see, e.g., Refs. 15,16). The B2se structures employed here are shown in the SI (see Fig. ), and the phenyl radical, as the only open-shell B2se species is excluded and later separately analysed in Section 4.4. Barone and co-workers have built the B2se set by using B2PLYP/cc-pVTZ to compute vibrational corrections. This level of theory gives nearly identical structures to those computed from CCSD(T) vibrational corrections. For a subset of the B2se structures, vibrational correction have been computed at the CCSD(T) with at least a triple-ζ basis set and these are contained in the CCse set. To validate the use of the B2se structures as a reference, we calculate GEO values for a range of approximate structures by using both B2se and CCse geometries as a reference. This is done for a subset of the B2se set for which the CCse geometries are available. The two sets of GEO values (B2se vs. CCse as a reference) are essentially the same (typically within 0.005 kcal/mol), as shown in Fig. of the SI. This confirms the suitability of the B2se structures as reference geometries for performing the GEO analysis in the present work.