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.