Do composite methods achieve their target accuracy?

- ccCA achieves an MAD of 0.80 kcal mol−1 for the ΔHf's of the G2/97 molecule set.
- CCSD(T,FC1)/aug-cc-pCV∞Z-DK has an MAD of 0.87 kcal mol−1 for the G2/97 set.
- ccCA can result in >90% computational time savings over CCSD(T).

Enthalpies of formation have been calculated for the 147 molecules of the G2/97 data set using the correlation consistent Composite Approach (ccCA) to assess the ability of this approach to achieve its presumed CCSD(T)-level accuracy. The calculated enthalpies of formation from the G2/97 data set were compared to enthalpies computed at the CCSD(T,FC1)/aug-cc-pCV∞Z-DK level of theory. Deviations of both ccCA and CCSD(T) enthalpies from experiment have been evaluated. ccCA results in a mean absolute deviation (MAD) of 0.80 kcal mol−1 from experiment while CCSD(T) has an MAD of 0.87 kcal mol−1. The MAD of the ccCA enthalpies from the CCSD(T,FC1)/aug-cc-pCV∞Z-DK enthalpies is 0.84 kcal mol−1.

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