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The theory of two-electron systems is extended to include d⁹p and d⁹d, and comparison is made with experimental data for the 3d⁹4p and 3d⁹4d configurations of Ni I. Greater discrepancies appear here than in d⁹s or p⁵p, but the multiplets are placed in the correct order. Though the neglected interactions are important, yet the values obtained for the parameters of the largest interactions seem not to be seriously disturbed.
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H. H. Marvin (1933) studied this question.
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