Large-basis shell model studies of light nuclei with a multi-valued G-matrix effective interaction

Large-basis shell model studies of low-lying excitations in light nuclei from 4He to 7Li have been performed with a multi-valued G-matrix effective interaction, as recently suggested by Haxton et al.. Calculations were performed relative to the vacuum (``no core") using very large, separable model spaces containing all excitations with unperturbed energies up to 8\hbar\Omega. Using G matrices derived from a new Nijmegen potential, we achieve a very satisfactory description of these excitations.