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.

References in corpus (4)

• Localization and delocalization errors in density functional theory and implications for band-gap prediction
• Fractional spins and static correlation error in density functional theory
• Eliminating spin contamination in auxiliary-field quantum Monte Carlo: realistic potential energy curve of F2
• Ground state of two electrons on a sphere

Cited by in corpus (2)

• Holomorphic Hartree-Fock Theory: The Nature of Two-Electron Problems
• Variations of the Hartree-Fock fractional-spin error for one electron