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Excitation energies and electromagnetic properties of the low energy spectra of21Ne and25Mg have been calculated using the Multi-Configuration-Hartree-Fock (MCHF) model. BothT=0 andT=1 pairing correlations are found to be simultaneously important in those odd mass nuclei. Furthermore, though axial symmetry was requested, quite good agreement with the experimental data is reached.

Shell-model and multi-configuration Hartree-Fock (MCHF) calculations in an Od 5 2 -1s 1 2 oscillator basis are compared for 20 Ne, 22 Ne and 24 Mg. The similarity of the calculated results in the two models shows that the MCHF method is a very good approximation to the shell-model calculations.

Multi-Configuration-Hartree-Fock (MCHF) calculations with angular momentum projection before the variation of the internal degrees of freedom have been performed for the nuclei20Ne and22Ne. This procedure yields different correlated intrinsic states for the different members of a rotational band. Thus the angular momentum dependence of correlations has been studied. Experimentally, the ground state spectra of20Ne and22Ne show properties similar to the phase transitions observed in some rare earth nuclei which have been well reproduced through the present calculations. The calculated spectra andBE2-values show a significant improvement compared to the ones obtained by variation before the angular momentum projection is effected.

In addition to the quartet excitations between major oscillator shells (intershell quartet states), we propose quartet excitations within a shell (intrashell quartet states). A phenomenological quartet shell model is devised to predict the energies of these states. Evidence for such excitations is also found in a microscopic multiconfiguration Hartree-Fock calculation.

The multi-configuration Hartree-Fock (MCHF) theory, a variational method for the self-consistent solution of the nuclear many-body problem with the inclusion of correlations, is applied to 12 C. The force is the Yale potential in the form of the Yale-Shakin matrix elements. 40 different configurations are included. The gain in the total binding energy compared to HF in the same single-particle basis and with the same force is 1.20 MeV. The MCHF theory yields the first excited 0 + state and the first excited 2 + state in good agreement with the experimental data.

The Multi Configuration Hartee-Kock (MCHF) theory is applied to the ground state of 28 Si. The single particle basis states are the 0 d 5 2 and 1 s 1 2 orbitals. A comparison is made with the results of a Hartree-Fock calculation performed in the same space.