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An analytical study of the 1S ground state of helium-like ions employing the open-shell function (1s1s′) and the angularly correlated configuration-interaction function (1s1s′)+λ(2p)2 has been made. Expressions for the wave function parameters, energy values, and certain other expectation values in terms of the nuclear charge Z have been obtained. These are asymptotic expressions for large Z, but they reproduce numerically obtained results for small Z with sufficient accuracy to be useful for many purposes.
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Scherr et al. (1960) studied this question.
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