  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 V8' 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 operator Nsps that measures
the harmonic-oscillator quanta of the spectator nucleons.
