Two methods for estimating the HF CBS limit using F12 specific basis sets are described in Section III above; CBS(T,Q), which uses a CBS extrapolation formula, and QZ+CABS, which relies on the CABS singles correction. To first calibrate the performance of CBS(T,Q), estimates of the HF/CBS limit are calculated for the neutral ground-state atoms of the 18 heavy post-d elements, and compared to the equivalent CBS limits estimated using Eqn. 2 with the conventional aug-cc-pVQZ-PP and aug-cc-pV5Z-PP basis sets. The mean unsigned deviation between these two estimates is 17 µE h , with a mean signed deviation of -10 µE h , where negative indicates that the CBS(T,Q) estimate produces a lower energy value.