title: Qualitative breakdown of the unrestricted Hartree-Fock energy.
abstract: The stretching of closed-shell molecules is a qualitative problem for restricted Hartree-Fock that is usually circumvented by the use of unrestricted Hartree-Fock (UHF). UHF is well known to break the spin symmetry at the Coulson-Fischer point, leading to a discontinuous derivative in the potential energy surface and incorrect spin density. However, this is generally not considered as a major drawback. In this work, we present a set of two electron molecules which magnify the problem of symmetry breaking and lead to drastically incorrect potential energy surfaces with UHF. These molecules also fail with unrestricted density-functional calculations where a functional such as B3LYP gives both symmetry breaking and an unphysically low energy due to the delocalization error. The implications for density functional theory are also discussed.
authors.affiliation: Departamento de Química and Instituto de Física de la Materia Condensada (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain.
authors.forename: Paula
authors.initials: P
authors.lastname: Mori-Sánchez
authors.affiliation: Department of Chemistry, Lensfield Road, University of Cambridge, Cambridge CB2 1EW, United Kingdom.
authors.forename: Aron J
authors.initials: AJ
authors.lastname: Cohen