It is pointed out that the perturbation expansions of unrestricted Hartree-Fock orbitals, based on zero-order uncorrelated orbitals, contain terms of order Z-1/2 which do not appear in the similar expansions of Hartree-Fock orbitals. It is shown that these terms give about 90% of the total radial correlation in the second-order energy of the 1S ground state of the two-electron systems.
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A L Stewart (1964) studied this question.
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