A functional renormalization group approach to electronic structure calculations for systems without translational symmetry

TL;DR

This paper introduces an energy-domain functional renormalization group ({ extepsilon}FRG) method for electronic-structure calculations applicable to inhomogeneous systems, improving upon existing approaches by self-consistently solving the Bethe-Salpeter equation and analyzing phase boundaries.

Contribution

It develops a novel { extepsilon}FRG formalism organized in energy space, extending applicability to disordered and inhomogeneous systems and surpassing GW-BSE accuracy.

Findings

Validated against exact diagonalization and DMRG results.

Successfully mapped phase boundary of disordered Hubbard model.

Demonstrated potential for finite temperature and spin extensions.

Abstract

A formalism for electronic-structure calculations is presented that is based on the functional renormalization group (FRG). The traditional FRG has been formulated for systems that exhibit a translational symmetry with an associated Fermi surface, which can provide the organization principle for the renormalization group (RG) procedure. We here advance an alternative formulation, where the RG-flow is organized in the energy-domain rather than in k-space. This has the advantage that it can also be applied to inhomogeneous matter lacking a band-structure, such as disordered metals or molecules. The energy-domain FRG ({\epsilon}FRG) presented here accounts for Fermi-liquid corrections to quasi-particle energies and particle-hole excitations. It goes beyond the state of the art GW-BSE, because in {\epsilon}FRG the Bethe-Salpeter equation (BSE) is solved in a self-consistent manner. An efficient implementation of the approach that has been tested against exact diagonalization calculations and calculations based on the density matrix renormalization group is presented. Similar to the conventional FRG, also the {\epsilon}FRG is able to signalize the vicinity of an instability of the Fermi-liquid fixed point via runaway flow of the corresponding interaction vertex. Embarking upon this fact, in an application of {\epsilon}FRG to the spinless disordered Hubbard model we calculate its phase-boundary in the plane spanned by the interaction and disorder strength. Finally, an extension of the approach to finite temperatures and spin S = 1/2 is also given.