title: Conditions for Describing Triplet States in Reduced Density Matrix Functional Theory.
abstract: We consider necessary conditions for the one-body reduced density matrix (1RDM) to correspond to a triplet wave function of a two-electron system. The conditions concern the occupation numbers and are different for the high spin projections, Sz = ±1, and the Sz = 0 projection. Hence, they can be used to test if an approximate 1RDM functional yields the same energies for both projections. We employ these conditions in reduced density matrix functional theory calculations for the triplet excitations of two-electron systems. In addition, we propose that these conditions can be used in the calculation of triplet states of systems with more than two electrons by restricting the active space. We assess this procedure in calculations for a few atomic and molecular systems. We show that the quality of the optimal 1RDMs improves by applying the conditions in all the cases we studied.
authors.affiliation: Max Planck Institute for the Structure and Dynamics of Matter , Luruper Chaussee 149, 22761 Hamburg, Germany.|Peter Grünberg Institut and Institute for Advanced Simulation, Forschungszentrum Jülich , D-52425 Jülich, Germany.
authors.forename: Iris
authors.initials: I
authors.lastname: Theophilou
authors.affiliation: Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation , Vass. Constantinou 48, GR-11635 Athens, Greece.|Max-Planck-Institut für Mikrostrukturphysik , Weinberg 2, D-06120 Halle (Saale), Germany.
authors.forename: Nektarios N
authors.initials: NN
authors.lastname: Lathiotakis
authors.affiliation: Peter Grünberg Institut and Institute for Advanced Simulation, Forschungszentrum Jülich , D-52425 Jülich, Germany.
authors.forename: Nicole
authors.initials: N
authors.lastname: Helbig