The pair-coupling model

The pair-coupling model is developed in the coherent-state representation and examined in some detail for a system of particles interacting with a pairing force. The pair-coupling model is equivalent to number-projected BCS theory, or more generally, to number-projected Hartree-Fock Bogoliubov theory, with variation after projection. The conservation of particle number comes at the expense of losing some of the simple features of the BCS quasiparticle approximation, associated with fermion second quantization. However, other useful properties are gained from the coherent-state representation and the result is a theory of pair correlations that is remarkably accurate as well as tractable and elegant.