Density-Corrected Density Functional Theory for Open Shells: How to Deal with Spin Contamination

Density functional theory (DFT) is usually used self-consistently
to predict chemical properties, but the use of the Hartree–Fock
(HF) density improves energetics in certain, well-characterized cases.
Density-corrected (DC) DFT provides the theory behind this, but unrestricted
Hartree–Fock (UHF) densities yield poor energetics in cases
of strong spin contamination. Here we compare with restricted open-shell
HF (ROHF) across 13 different functionals and two DC-DFT methods.
For significant spin contamination, ROHF densities outperform UHF
densities by as much as a factor of 3, depending on the energy functional,
and ROHF-DFT improves over self-consistent DFT for most of the tested
functionals. We refine the DC(HF)-DFT algorithm to use ROHF densities
in cases of severe spin contamination.