Interpretations of ground-state symmetry breaking and strong correlation in wavefunction and density functional theories

TL;DR

This paper explores how approximate density functional theories can reveal symmetry-breaking phenomena in strongly correlated systems, illustrating their implications through examples like stretched H2, antiferromagnetic solids, and charge-density waves.

Contribution

It demonstrates that symmetry-breaking in approximate density functionals can provide insights into strong correlations and collective excitations, contrasting with the symmetry-preserving exact functionals.

Findings

Symmetry-breaking in DFT can reveal strong correlation effects.

Charge density waves can be interpreted as soft plasmons.

Approximate functionals may be more revealing despite less accuracy.

Abstract

Strong correlations within a symmetry-unbroken ground-state wavefunction can show up in approximate density functional theory as symmetry-broken spin-densities or total densities, which are sometimes observable. They can arise from soft modes of fluctuations (sometimes collective excitations) such as spin-density or charge-density waves at non-zero wavevector. In this sense, an approximate density functional for exchange and correlation that breaks symmetry can be more revealing (albeit less accurate) than an exact functional that does not. The examples discussed here include the stretched H$_2$ molecule, antiferromagnetic solids, and the static charge-density wave/Wigner crystal phase of a low-density jellium. It is shown that (and in what sense) the static charge density wave is a soft plasmon.