  We have calculated the ground-state energy of the doubly-magic nucleus 40Ca
within the framework of the Goldstone expansion using the CD-Bonn
nucleon-nucleon potential. The short-range repulsion of this potential has been
renormalized by integrating out its high-momentum components so as to derive a
low-momentum potential V-low-k defined up to a cutoff momentum Lambda. A simple
criterion has been employed to establish a connection between this cutoff
momentum and the size of the two-nucleon model space in the harmonic oscillator
basis. This model-space truncation approach provides a reliable way to
renormalize the free nucleon-nucleon potential preserving its many-body
physics. The role of the 3p-3h and 4p-4h excitations in the description of the
ground state of 40Ca is discussed.
