Potential energy surfaces from many-body functionals: analytical benchmarks and conserving many-body approximations

TL;DR

This paper evaluates the performance of many-body energy functionals, derived from Klein and Luttinger-Ward, on a solvable Hubbard model to benchmark their accuracy and computational efficiency in describing strongly correlated electrons.

Contribution

It provides analytical benchmarks for many-body functionals on the Hubbard model and demonstrates the Luttinger-Ward functional's potential as an efficient alternative to self-consistent calculations.

Findings

Luttinger-Ward functional closely matches fully self-consistent results.

Both functionals show good variational properties, especially Luttinger-Ward.

Proper choice of input Green's functions enhances accuracy and usefulness.

Abstract

We investigate analytically the performance of many-body energy functionals, derived respectively by Klein and Luttinger and Ward, at different levels of diagrammatic approximations, ranging from second Born, to GW, to the so-called T-matrix, for the calculation of total energies and potential energy surfaces. We benchmark our theoretical results on the extended two-site Hubbard model, which is analytically solvable and for which several exact properties can be calculated. Despite its simplicity, this model displays the physics of strongly correlated electrons: it is prototypical of the H$_2$ dissociation, a notoriously difficult problem to solve accurately for the majority of mean-field based approaches. We show that both functionals exhibit good to excellent variational properties, particularly in the case of the Luttinger-Ward one, which is in close agreement with fully self-consistent calculations, and elucidate the relation between the accuracy of the results and the different input one-body Green's functions. Provided that these are wisely chosen, we show how the Luttinger-Ward functional can be used as a computationally inexpensive alternative to fully self-consistent many-body calculations, without sacrificing the precision of the results obtained. Furthermore, in virtue of this accuracy, we argue that this functional can also be used to rank different many-body approximations at different regimes of electronic correlation, once again bypassing the need for self-consistency.