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Abstract A method for direct configuration–interaction ( CI ) calculations with a multiconfigurational reference function is described. The reference state can contain several closed‐shell electronic configurations and the CI expansion comprises all single and double replacements out of all these configurations. The resulting secular problem is solved using a variation‐perturbation method. A number of examples are given showing the efficiency of the method. The largest CI expansion used in calculations with this program so far contains 76,471 spin‐ and space‐symmetrized configurations.
Roos et al. (Sat,) studied this question.