The errors in the HOFs for PM6 (and all other methods) relative to G4 are listed in Table , and show that the error in the HOFs relative to G4 are CH 4 : -5. In contrast, the PBEh-3c error for the parent reaction is 9.0 kcal/mol, which decreases to -4.5 kcal/mol with the CBH-1 correction. The error in the HOFs relative to G4 are CH 4 : 8.5, CH 2 --CH 2 : 6.0, and CH 3 CH 3 : 14.9 kcal/mol. So the CBH-1 correction is 4(14.9) -2(6.0) -4(8.5) = 13.6 kcal/mol, which when subtracted from 9.9 lowers the error in reaction energy to -4.5 kcal/mol. Clearly the errors in HOFs are much larger than for PM6, yet most of the error cancels. The reason is that the magnitude of the HOF error tends to scale with the number and types of bonds in the molecule for ab initio methods. For example, the HOF-error of CH 2 --CHCH 3 is 13.7 kcal/mol, which is approximately the sum of the HOF-errors of CH 3 CH 3 and CH 2 --CH 2 , minus the error for CH 4 : 14.9 + 6.0 -8.5 = 12.4 kcal/mol. This type of error scaling makes ab initio results very amenable to improvement using the CBH approach.