Staying with the differences between the He and Be isoelectronic series discussed earlier, Fig. compares the reference (FCI) correlation energies and those from Padé interpolation (resulting from Eq. 43) for both series as Z increases. Several key differences are evident. First, for the He series, the reference E c tends to a constant with increasing Z, whereas for the Be series, E c becomes increasingly negative. Second, MP2 results become more accurate with increasing Z for the He series-consistent with the near-linearity of E λ c for large-Z ions (Fig. )-whereas for the Be series, MP2 accuracy deteriorates as Z increases. Finally, for the He series, the Padé interpolation markedly outperforms MP2, yielding excellent accuracy for Z ≥ 2. For the Be series, Padé still improves upon MP2, yet insufficiently.