We start by considering the PM6 result for the first reaction (R1), where the PM6 error is -0.3 kcal/mol for the parent reaction and the CBH-1 correction is -12.7 kcal/mol. PM6 error is quite low for the parent reaction so, ideally, the CBH-1 correction should be close to zero, but is in fact quite large in magnitude. In order to understand the source of this error we reformulate the (∆CBH-n(G4) -∆CBH-n(LL)) correction in terms of errors in heats of formation (HOF, ∆H