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The correlation energy for the ground-state isoelectronic series of the second-row atoms of the periodic table are estimated. In this estimate the relativistic energies for the closed and the open shells, and the Hartree—Fock energies are taken from calculations we have recently performed. As expected, the second-row correlation energy parallels, in general, the characteristics of the first-row correlation energy. The principal contribution to the correlation energy is due to pairing of electrons in a given orbital. The intershell correlation is smaller than the intrashell correlation (but not negligible) and the 3s—3s electron correlation is linear in Z.
E. Clementi (Mon,) studied this question.