An additional complication with GTO bases is that their intrinsic construction is not necessarily systematic as their size increases, which possibly leads to difficulties in converging properties in a smooth and monotonic way, which is a known problem for Hartree-Fock (HF) or correlated methods such as e.g. second-order Møller Plesset perturbation (MP2) or coupled cluster (CC) energies. For this reason, Dunning's cc-pVXZ correlation-consistent polarized bases (where X=D,T,Q,5,6 is the cardinal number), as well as their augmented aug-cc-pVXZ analogues containing diffuse functions , are among the most popular for post-HF approaches because of their meticulous design which makes it possible to gradually recover a maximum of electron correlation by increasing the basis' cardinality.