As a second technical aspect, we note that the standard GGA functional often fails to accurately describe the exact ground state for iron due to the spatial compactness of the d-orbitals. Including an on-site Coulomb repulsion term U in calculations is one way to properly handle 3d orbitals. This was recently discussed by us in detail for FeNC models. For selected cases below, we explore the use of GGA+U and the incorporation of exact Hartree-Fock exchange (HFE) in the PBE0 hybrid functional.