Identifying strong correlation using only the Kohn-Sham density of one-electron states

Abstract

Strongly correlated systems have long been a central and highly non-trivial topic in condensed matter physics. At the non-interacting level, strong correlation can be associated with powerful (near) degeneracies between occupied and unoccupied states, which leads to a high density of states near the Fermi level in metallic configurations. Such regimes are commonly treated with beyond-density functional theory (DFT) approaches, such as DFT+U or DFT+DMFT while maintaining symmetric configurations. Here, we explore the hypothesis that symmetry breaking in the Kohn-Sham (KS) non-interacting system can qualitatively account for the energetic effects of strong correlation in the corresponding interacting system within standard DFT. By lifting near-degeneracies around the Fermi level, symmetry breaking diminishes the potential correlation effects, reducing the need for an explicit treatment of electron correlation, transforming an otherwise strongly correlated symmetric configuration into a normally correlated one, thus avoiding the need for interacting methods beyond DFT. This naturally connects nonmagnetic to magnetic states. We apply this idea to both strongly and normally correlated metals and observe that spin symmetry breaking leads to a pronounced reduction of the density of states at the Fermi level and a significant lowering of the total energy in strongly correlated cases. To describe the degree of correlation that the interacting system would have relative to the KS state, we introduce a correlation parameter ($\Gamma$), defined as the ratio between the Kohn-Sham density of one-electron states at the Fermi level and that of a corresponding uniform electron gas. This parameter distinguishes strongly correlated systems, which would require explicit treatment, from normally correlated ones, which do not.