ar X iv : n uc l - th / 0 11 20 66 v 1 2 1 D ec 2 00 1 Ab − initio No - Core Shell Model With Many - Body Forces

Working in the framework of the ab−initio no-core shell model, we derive two-body effective interactions microscopically for specific harmonic-oscillator basis spaces from the realistic Argonne V 8 ′ nucleon-nucleon potential. However, our two-body effective interaction is only the dominant part of the A−body (A ≥ 3) effective interaction, which also contains three-, four-and, in general, up to A−body components. We model these multi − body components as a simple two-parameter, basis-space-dependent interaction. We then investigate the coupling needed to provide accurate descriptions of selected nuclear observables. We successfully model the multi-body interaction as a zero-range two-body operator times the number operatorˆNsps, that measures the harmonic-oscillator quanta of the spectator nucleons.