NATURAL ORBITALS, DIVERGENCES, AND VARIATIONAL PRINCIPLES

Bogoliubov's principle of compensation of dangerous diagrams (PCDD) is extended to finite fermion systems and is shown to give Löwdin's natural spin-orbitals (NSO) which diagonalize the single-particle reduced density matrix. Hartree-Fock theory corresponds to the compensation of the lowest-order dangerous diagrams, which cause divergences in the perturbation expansion of the ground-state energy. The PCDD is then derived from a variational principle that the number of particle-hole excitations in the true ground state is a minimum. The sense in which the NSO give the most rapid rate of convergence of the configuration-interaction expansion of the true wavefunction is also discussed.