The difference between molecules and materials: Reassessing the role of exact conditions in density functional theory

TL;DR

This paper reevaluates the importance of exact conditions in density functional theory, highlighting that many empirical approximations satisfy these conditions for realistic densities, and discusses their role in developing better functionals.

Contribution

It distinguishes necessary versus sufficient conditions and explores how empirical and non-empirical approximations relate to these conditions in DFT.

Findings

Many empirical approximations satisfy multiple exact conditions for realistic densities.

Non-empirical approximations satisfy more conditions than those explicitly enforced.

Revives obscure exact conditions and clarifies their role in functional development.

Abstract

Exact conditions have long been used to guide the construction of density functional approximations. But hundreds of empirical-based approximations tailored for chemistry are in use, many of which neglect these conditions in their design. We analyze well-known conditions and revive several obscure ones. Two crucial distinctions are drawn: that between necessary and sufficient conditions, and between all electronic densities and the subset of realistic Coulombic ground states. Simple search algorithms find that many empirical approximations satisfy many exact conditions for realistic densities and non-empirical approximations satisfy even more conditions than those enforced in their construction. The role of exact conditions in developing approximations is revisited.