Variational theory combining number-projected BCS and coupled-cluster doubles

TL;DR

This paper introduces a variational method combining number-projected BCS and coupled-cluster doubles to accurately describe pairing correlations in finite fermionic systems across different coupling regimes.

Contribution

It presents a novel symmetry-preserving wavefunction approach that unifies weak and strong coupling regimes with high accuracy and provides an energy upper bound.

Findings

Achieves high-precision energy estimates comparable to non-variational methods.

Provides a unified framework valid from weak to strong coupling.

Operates effectively in both small and large systems.

Abstract

The ground state pairing correlations in finite fermionic systems are described with a high degree of accuracy within a variational approach based on a combined coupled-cluster and particle-number-projected BCS ansatz. The flexibility of this symmetry-preserving wavefunction enables a unified picture valid from weak to strong coupling, both in small and large systems. The present variational approach consistently yields an energy upper bound while operating at the same level of precision of the non-variational particle-number projected Bogoliubov-coupled-cluster theory [Phys. Rev. C 99, 044301 (2019)].