  Some of the most spectacular failures of density-functional and Hartree-Fock
theories are related to an incorrect description of the so-called static
electron correlation. Motivated by recent progress on the N-representability
problem of the one-body density matrix for pure states, we propose a way to
quantify the static contribution to the electronic correlation. By studying
several molecular systems we show that our proposal correlates well with our
intuition of static and dynamic electron correlation. Our results bring out the
paramount importance of the occupancy of the highest occupied natural
spin-orbital in such quantification.
