  An exact relation which links the ideal model space to be used in A-body
calculations when the two-body interaction is given in a truncated model space
is derived. Its implications on the effective field theory (EFT) approach to
no-core-shell-model (NCSM) is analyzed. Some insights regarding whether details
of two-body interaction becomes less important in the calculations of many-body
system are given. The result suggests that there might be a way to establish an
EFT expansion for heavy nuclei and nuclear matter with an effective
intereaction which has a much simpler form than the nucleon-nucleon (NN)
interaction in the vacuum.
