Density functional versus spin-density functional and the choice of correlated subspace in multi-variable effective action theories of electronic structure

TL;DR

This paper compares different choices in extended density functional theories, focusing on the impact of using charge versus spin densities and different orbital definitions on the accuracy and physicality of the results, especially in rare earth nickelates.

Contribution

It provides a formal and practical comparison of charge and spin density functionals and orbital definitions in DFT+U, highlighting their effects on structural energetics and effective interactions.

Findings

Spin-dependent functionals can produce unphysical exchange couplings.

Orbital definitions yield similar results at ambient pressure but differ at high pressure.

Implications for DFT+DMFT and other beyond-DFT methods are discussed.

Abstract

Modern extensions of density functional theory such as the density functional theory plus U and the density functional theory plus dynamical mean-field theory require choices, including selection of variable (charge vs spin density) for the density functional and specification of the correlated subspace. This paper examines these issues in the context of the "plus U" extensions of density functional theory, in which additional correlations on specified correlated orbitals are treated using a Hartree-Fock approximation. Differences between using charge-only or spin-density-dependent exchange-correlation functionals and between Wannier and projector-based definitions of the correlated orbitals are considered on the formal level and in the context of the structural energetics of the rare earth nickelates. It is demonstrated that theories based on spin-dependent exchange-correlation functionals can lead to large and in some cases unphysical effective on-site exchange couplings. Wannier and projector-based definitions of the correlated orbitals lead to similar behavior near ambient pressure, but substantial differences are observed at large pressures. Implications for other beyond density functional methods such as the combination of density functional and dynamical mean field theory are discussed.