  In this work we investigate the so-called model-space Brueckner-Hartree-Fock
(MBHF) approach for nuclear matter as well as for neutron matter and the
extension of this which includes the particle-particle and hole-hole (PPHH)
diagrams. A central ingredient in the model-space approach for nuclear matter
is the boundary momentum $k_M$ beyond which the single-particle potential
energy is set equal to zero. This is also the boundary of the model space
within which the PPHH diagrams are calculated. It has been rather uncertain
which value should be used for $k_M$. We have carried out model-space nuclear
matter and neutron matter calculations with and without PPHH diagrams for
various choices of $k_M$ and using several modern nucleon-nucleon potentials.
Our results exhibit a saturation region where the nuclear and neutron matter
matter energies are quite stable as $k_M$ varies. The location of this region
may serve to determine an "optimum" choice for $k_M$. However, we find that the
strength of the tensor force has a significant influence on binding energy
variation with $k_M$. The implications for nuclear and neutron matter
calculations are discussed.
