On the performance of density functional theory for symmetry-breaking problems

Density functional theory (DFT) has been applied to three open-shell molecules (NO3, O4+, and O2+) for which the unrestricted Hartree–Fock (UHF) wavefunction breaks spatial symmetry. In contrast to Hartree–Fock, all of the standard DFT methods we employed yielded symmetric densities for each of the molecules considered. Symmetry-broken solutions were obtained with DFT only when we used hybrid functionals including unusually large fractions of Hartree–Fock exchange. The exchange functional seems more important than the correlation functional in determining whether symmetry is preserved or broken.