  Effective interactions can be obtained from a renormalization group analysis
in two complementary ways. One can either explicitly integrate out higher
energy modes or impose given conditions at low energies for a cut-off theory.
While the first method is numerically involved, the second one can be solved
almost analytically. In both cases we compare the out coming effective
interactions for the two nucleon system as functions of the cut-off scale and
find a strikingly wide energy region where both approaches overlap,
corresponding to relevant scales in light nuclei about 200MeV. This amounts to
a great simplification in the determination of the effective interaction
parameters.
