When is an atom heavy Ionization potentials of alkali atoms

When is an atom heavy Ionization potentials of alkali atoms 

Alkali atoms are basically one-valence-electron systems, and correlation would therefore not be expected to play a major role in the determination of properties such as ionization potentials. Nevertheless, a Hartree-Fock (or, for the heavier alkali atoms, a Dirac-Fock) calculation gives sizable errors for this property. In particular, if accuracy below the one percent level 

Total energies of the closed-shell systems AU+ and Au (hartree), with nonrelativistic, DC, and DCB Hamiltonians. Correlation includes 4s and higher shells. 

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