... for the spin-summed singlet case, where nj ðr; tÞ are natural
orbitals. The N-representability conditions are that 0 gj 2,
j gj 5N. (For the spin-resolved case, the first condition
becomes 0 gj 1). The 1RDM should be positive semidefinite, with trace equal N, and each eigenvalue bounde ...