Approximating one-matrix functionals without generalized Pauli constraints

Common features are identified in the properties of the pure (PF) and ensemble (EF) energy functionals defined within the density matrix functional theory (DMFT) as well as their free-form (FFF) approximations. These features rationalize abandoning, in the common DMFT practice, prohibitively complicated generalized Pauli constraints (GPCs). Specifically, it is shown that the exact PF and EF coincide on the set of the v-representable first-order reduced density matrices (1RDMs), while FFFs approximate the common PF and EF for such 1RDMs. It is revealed that for some types of FFFs, for example for the geminal-based approximations, the GPCs are essentially trivialized to the pairwise natural occupation numbers (NONs) degeneracy conditions, routinely imposed for closed-shell systems.