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

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