Generalized Hartree-Fock Description of Molecular Dissociation

An electronic structure method is said to be size-consistent if the energy of noninteracting fragments is the same when the fragments are treated in a supermolecule approach or are treated in isolation. Size consistency is often violated by Hartree-Fock when symmetries of the exact wave function are imposed on the Hartree-Fock determinant. Relaxing the requirement that the Hartree-Fock wave function be a spin eigenfunction leads to unrestricted Hartree-Fock, which is often (but not always) size-consistent. In this Perspective, we discuss the usually forgotten fact that imposing none of the exact symmetries in what is known as generalized Hartree-Fock allows Hartree-Fock to always be size-consistent and allows size extensive correlated methods such as coupled cluster theory to also be size-consistent. Furthermore, with all symmetries broken, dissociation curves connect the molecule to the fragments better than with symmetries imposed, although the curves are not smooth and show derivative discontinuities akin to unphysical phase transitions. In many cases, correlated dissociation curves based on this generalized Hartree-Fock reference are discontinuous.