Unrestricted Hartree-Fock then and now

The unrestricted Hartree-Fock model of interacting electrons originated as an approximation that incorporates exchange-polarization effects for open-shell atoms and molecules. Spontaneous symmetry-breaking is described by this model, in particular for dissociating covalent bonds, but is limited to exchange in the Hartree-Fock context. When this model is extended to an orbital functional theory, which can include quantitative effects of electronic correlation, it becomes a widely applicable formalism for many-body theory. This conceptual extension is discussed. © 2004 Wiley Periodicals, Inc.