  Starting from a precise two-nucleon potential, we use the method of unitary
transformations to construct an effective potential that involves only momenta
less than a given maximal value. We describe this method for an S-wave
potential of the Malfliet-Tjon type. It is demonstrated that the bound and
scattering state spectrum calculated within the effective theory agrees exactly
with the one based on the original potential. This might open an avenue for the
construction of effective chiral few-nucleon forces and for a systematic
treatment of relativistic effects in few-body systems.
