was giving the best match for (α = 3, B = 0, D = 0) and (α = 4, C = 0, D = 1), consequently proposing Eqs. Wilson35 (26) and Wilson45 (27) for extrapolating correlated energies . Alternatively, Helgaker et al. put forward the use of Eq. Helgaker (29) which corresponds to the leading order of Eq.36 and the identification of L ∼ X -1 based on a better agreement with R12 results . This allows the extrapolation of the correlation energy from a two-point linear fit and was later motivated by analyzing the energy increments of the principal expansion of the ground-state helium atom, yielding in this case a convergence with respect to the principal quantum number n which is a priori more in line with the progressive construction of the (aug-)cc-pVXZ bases. Since the number of basis functions increases cubically with X (Eq.17), Helgaker (29) is equivalent to a convergence of the correlation energy as a function of 1/N b . Interestingly, this is analogous to PWs for which the correlation energy converges as 1/N G (Eq.10). A different rate of convergence was observed for the HF energy so that Helgaker argued for its separate treatment according to Helgaker (28) .