Symmetry broken and restored coupled-cluster theory: II. Global gauge symmetry and particle number
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, $\it J. Phys. G: Nucl. Part. Phys.$ 42, 025107 (2015). 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.
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