Reducing density-driven error without exact exchange

The errors in density functional theory (DFT) calculations can be decomposed into contributions from the exchange–correlation density functional approximation (DFA), and contributions from the approximate electron density generated by that DFA. Standard “semilocal” DFAs have large density-driven delocalization errors for dissociating bonds, radical complexes, metal–ligand complexes, reaction intermediates, and reaction barriers. Several recent studies use Hartree–Fock exchange to reduce these density-driven errors. However, Hartree–Fock calculations can be formally and computationally problematic in periodic systems. I show that Rung 3.5 DFAs, which project the Kohn–Sham one-particle density matrix onto a localized model density matrix at each point in space, can provide a practical alternative. Rung 3.5 densities reduce the aforementioned density-driven errors without empirical parametrization, without the orbital rotation dependence of self-interaction corrections, and without any exact exchange whatsoever. While existing Rung 3.5 DFAs cannot reduce density-driven errors as much as Hartree–Fock exchange, these results offer new prospects for broadening the reach of density-corrected DFT.