They found Δ = 1.36, 0.44, and 0.04 kcal/mol, for the X pairs (D,T), (T,Q), and (Q,5), respectively. Note that the reference values are supposed to represent CCSD(T)/CBS, not the exact answer (which would be all-electron configuration interaction at the CBS limit). A previous study of 48 reaction energies and barrier heights gave a mean absolute deviation of CCSD(T)/CBS energies from more accurate benchmarks (with beyond-CCSD(T) contributions) of 0.44 kcal/mol. We conclude that that including beyond-CCSD(T) correlation effects is often more important than improving the CCSD(T) extrapolation from (T,Q) to (Q,5). For this reason and because (Q,5) extrapolation is often unaffordable, the present study is limited to (D,T) and (T,Q).