Symmetry broken and restored coupled-cluster theory. II. Global gauge symmetry and particle number

TL;DR

This paper extends coupled-cluster and many-body perturbation theories to restore global gauge symmetry and particle number, enabling more accurate descriptions of quantum systems with broken symmetries.

Contribution

It introduces a formalism for restoring particle number symmetry within Bogoliubov coupled-cluster theory, complementing previous work on angular momentum restoration.

Findings

Formalism for particle number restoration developed

Potential to merge $SU(2)$ and $U(1)$ symmetry treatments

Future applications to open-shell quantum systems

Abstract

We have recently extended many-body perturbation theory and coupled-cluster theory performed on top of a Slater determinant breaking rotational symmetry to allow for the restoration of the angular momentum at any truncation order [T. Duguet, J. Phys. G: Nucl. Part. Phys. 42 (2015) 025107]. Following a similar route, we presently extend Bogoliubov many-body perturbation theory and Bogoliubov coupled cluster theory performed on top of a Bogoliubov reference state breaking global gauge symmetry to allow for the restoration of the particle number at any truncation order. Eventually, formalisms can be merged to handle $SU(2)$ and $U(1)$ symmetries at the same time. Several further extensions of the newly proposed many-body formalisms can be foreseen in the mid-term future. The long-term goal relates to the ab initio description of near-degenerate finite quantum systems with an open-shell character.