Independent electron pair approximation IEPA

This defines an independent electron pair approximation in terms of extremal pairs, which can be regarded as a generalization of the independent electron pair approximation (IEPA) [4, 8] in terms of pairs (ij) constructed form (preferably) localized orbitals. As in the discussion in Paper I for MP2 [5], one can show that the extremal pairs defined in this section are related to approximate natural geminals corresponding to the coupled-cluster wave function. 

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