It is commonplace to run CCSD(T) calculations with the frozen core (FC) approximation, and if this approximation is turned on for CCSD(T)/aug'-cc-pCV5Z, the structure is still accurate (green dot), but it is off the center. This confirms that apart from a large basis set one also needs all-electron CCSD(T) to rival the accuracy of the SE approach. 10 This also demonstrates the advantage of using the SE structures as running CCSD(T)(full) with a large basis can only be done for very small molecules. When it comes to the DFT approximations in the GEO contour plot, one can observe their clustering, as previously observed for water. Here, the hybrid functionals are clustered together in the first quadrant (too short triple bond and accurate single bond), and metaGGA/GGAs are in the second quadrant (predicting too long bonds with larger errors for the single bond). M06-L 19 is an exception, but as we shall see later, its performance for geometric structures is more in-line with hybrids than with metaGGAs.