Density functional theory (DFT) has been applied to three
open-shell molecules (NO_{3},
O_{4}^{+}, and O_{2}^{+})
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.