International Workshop on 'New challenges in Reduced Density Matrix Functional Theory: Symmetries, time-evolution and entanglement'
Spin and pure state N-representability constraints in reduced density matrix functional theoryNicole Helbig
Reduced Density Matrix Functional Theory is a method that relies on the 1-1 correspondence between the many-body ground-state wave function and the first order reduced density matrix (1RDM) and uses the latter as its fundamental variable. The ground state of a system is determined within this approach by minimizing the energy functional with respect to the 1RDM under the constraint that the 1RDM corresponds to a fermionic ensemble (Coleman's conditions). Additional constraints can be employed to ensure the existence of a fermionic ensemble with a specific S_z . However, there remain the two questions if the fermionic system corresponds to a specific total spin and if the system can be represented by a pure state. We show that generally the answer to both questions is negative unless additional constraints are enforced and discuss these constraints exemplary for several small systems.
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