Quasipinning and selection rules for excitations in atoms and molecules

Postulated by Pauli to explain the electronic structure of atoms and molecules, the exclusion principle es-tablishes an upper bound of 1 for the fermionic occupation numbers, {ni}. A recent analysis of the pure N-representability problem provides a wide set of inequalities for the {ni}, leading to constraints on these numbers. In this work we study the nature of these inequalities for some atomic and molecular systems. Our results suggest that saturation (i.e., the inequalities becoming equalities) of some of these inequalities leads to a strong selection rule for the dominating configurations in configuration interaction (CI) expansions, which ultimately can provide means for significantly reducing their computational requirements. PACS numbers: 31.15.V-, 03.67.-a, 05.30.Fk I