  We perform large-space shell-model calculations for the low-lying energy
spectra of a few light nuclei, $^4\mbox{He}$, $^5\mbox{He}$, $^6\mbox{Li}$ and
$^7\mbox{Li}$, in a no-core model space with a realistic effective two-body
interaction (Brueckner G-matrix). Our G-matrices are calculated for the
Reid-soft-core potential in a harmonic oscillator basis. Single-particle ``-U''
insertions are replaced by two-particle ``-U'' insertions and are included in
the G matrix calculations, which sum them to all orders. With the starting
energy of the G-matrix chosen to give approximately the experimental binding
energy, we obtain nuclear energy spectra which are in reasonable agreement with
experiment. We also investigate the dependence of our results on the size of
the model space and on the harmonic oscillator basis parameter ($\hbar\Omega$).

Nuclear Theory