Natural Orbitals, Divergences, and Variational Principles

Abstract

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.