On number and angular momentum projection from Hartree-Bogoliubov states

Rotational spectra, calculated by angular momentum projection from Hartree-Bogoliubov states may be completely distorted by particle number nonconservation. A simple method for correcting this is presented, which brings them close to the number projected spectra. It is also shown that by a slight modification of the usual number projection operator this projection may then be executed two times faster and with better numerical accuracy. We study the effect ofJ- and/orN projection before the variation in a constrained Hartree-Bogoliubov model. It is demonstrated that only simultaneous projection of both particle number and angular momentum before the variation is meaningful. Then a more gradual antipairing effect is found than known from previous work. We conclude however that the diagonalization of the Hamiltonian in a space of appropriately chosen generator wave functions is preferable to projection before the variation. In all cases the examples are nuclei in thesd-shell, calculated selfconsistently without separating off and inert core. The nucleon-nucleon force is the Hamada-Johnston potential.