  A systematic numerical investigation of a recently developed nuclear
structure approach is presented which diagonalizes the Hamiltonian in the space
of the symmetry-projected Hartree-Fock-Bogoliubov (HFB) vacuum and
symmetry-projected quasiparticle excitations with respect to it. The underlying
HFB transformation, which is assumed to be time-reversal and axially symmetric,
is determined by variation after the projection. The model allows the use of
large basis systems. It has been applied to the calculation of energy spectra
of several even-even, odd-odd and odd mass nuclei in the sd shell with mass
numbers reaching from A=20 to 30. The Chung-Wildenthal interaction has been
used. Good agreement with the exact shell model diagonalization and a
considerable improvement on a previous approach, where the HFB transformation
was significantly more restricted, is obtained.
