In a recent international collaboration with Yonsei university (Seoul) and Argonne national labs, Burke's group has shown, using very demanding Quantum Monte Carlo simulations, that standard approaches for the spin gap in small Fe(II) complexes (both ab initio and DFT) are inaccurate. They also find that DC-DFT, using HF densities, works well, and so should be used instead of standard DFT for applications.

For more, see https://pubs.acs.org/doi/10.1021/acs.jctc.7b01196