Both contain three parameters and require at least three data points for extrapolation. The power expression of Truhlar with D, T, and Q (DTQ) data points generally worsens the deviations from PWs compared to simple Q energies. When including the 5 point (TQ5, DTQ5), results are also worse than or comparable to the plain 5 values in terms of MAE and maximum deviation for both non-augmented and augmented bases. For Helgaker, DTQ points improve the convergence of uncorrected and half-CP Q zeta energies (Fig. , ∆E HF ) but slightly deteriorate those that are CP corrected. The TQ5 results are essentially similar to the plain 5 zeta energies, and all extrapolated values are either slightly better or similar when considering all DTQ5 points. Thus, Helgaker (28) applied with the CP correction always gives the best match with PW HF energies as summarized in Table . From this observa- tion, such an exponential expression is the most appropriate for extrapolating HF interaction energies, which are degraded if extrapolated with the scheme of Truhlar. Although its usefulness is rather marginal on relative energies that are essentially converged from the Q zeta level (c.f. Table ), the agreement or small improvement over the non-extrapolated results, and the quality of interpolation (Table ), support the fact that the total (absolute) HF energies can be accurately extrapolated with Helgaker (28). With respect to MP2 interaction energies, results show that a majority of GTO extrapolations generally induce larger deviations from PW values than results directly obtained with the highest X point in the fitting sequence. This is particularly the case for CP-corrected energies for which extrapolations are expected to perform well on the reminiscent BSIE effects, and happens for the expressions of Truhlar (22), Martinα (25), Wilson35 (26), Wilson45 (27), Varandas34 (30), Varandas3-fit (31) and Var.34-fit (32). These schemes can be therefore invalidated outright to accurately estimate GTO energies in the CBS limit and are left in the Supplementary Material for the interested reader's discretion (Fig. , S6, S7).