A variational trial wave function is used to develop a perturbation expansion for the effective shell model interaction in the case of a degenerate zero-order basis. The form of this trial function is suggested by the Brillouin-Wigner perturbation procedure, and the Moszkowski-Scott separation method. Application is made to the low lying levels of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$. The absolute binding energies obtained are 4 to 5 MeV short of experimental values. Acceptable agreement with experiment is found for the energy level spacing.

• Received 11 December 1984

DOI:https://doi.org/10.1103/PhysRevC.31.1533