A simple two-level model is developed and used to test the properties of
effective interactions for performing nuclear structure calculations in
truncated model spaces. It is shown that the effective many-body interactions
sensitively depend on the choice of the single-particle basis and they appear
to be minimized when a self- consistent Hartree-Fock basis is used.Comment: (15 pages of text and 1 postscript figure (Figure available upon
request), Preprint Number not assigned ye