Ground state correlations in the BCS treatment of a monopole pairing Hamiltonian

The treatment of a monopole pairing Hamiltonian, in the quasiparticle basis and with the inclusion of ground state correlations, is discussed. The theoretical procedure is based on the BCS method supplemented by a variational treatment of ground state correlations. A significant improvement in the results is found, for ground state energies, when the comparison between correlated, BCS and exact values is performed. Results are discussed for one and two level model configurations.