Landscape of an exact energy functional

TL;DR

This paper visualizes and analyzes the exact energy functional in a simplified two-site Hubbard model, revealing its landscape, non-convexity, and derivative discontinuities, advancing understanding of density functional theory.

Contribution

It provides the first complete visualization and analysis of the exact functional across all electron numbers, including fractional, in a simplified model.

Findings

Exact functional is a simple 2D function in the model.

The energy landscape's shape explains failures of approximate functionals.

Derivative discontinuity relates directly to the fundamental gap.

Abstract

One of the great challenges of electronic structure theory is the quest for the exact functional of density functional theory. Its existence is proven, but it is a complicated multivariable functional that is almost impossible to conceptualize. In this paper, the asymmetric two-site Hubbard model is studied, which has a two-dimensional universe of density matrices. The exact functional becomes a simple function of two variables whose three dimensional energy landscape can be visualized and explored. A walk on this unique landscape, tilted to an angle defined by the one-electron Hamiltonian, gives a valley whose minimum is the exact total energy. This is contrasted with the landscape of some approximate functionals, explaining their failure for electron transfer in the strongly correlated limit. We show concrete examples of pure-state density matrices that are not $v$-representable due to the underlying non-convex nature of the energy landscape. For the first time, the exact functional is calculated for all numbers of electrons, including fractional, allowing the derivative discontinuity to be visualized and understood. The fundamental gap for all possible systems is obtained solely from the derivatives of the exact functional.