  We present a new approach to the construction of effective interactions
suitable for many-body calculations by means of the no-core shell model (NCSM).
We consider an effective field theory (EFT) with only nucleon fields directly
in the NCSM model spaces. In leading order, we obtain the strengths of the
three contact terms from the condition that in each model space the
experimental ground-state energies of 2H, 3H and 4He be exactly reproduced. The
first (0^+;0) excited state of 4He and the ground state of 6Li are then
obtained by means of NCSM calculations in several spaces and frequencies. After
we remove the harmonic-oscillator frequency dependence, we predict for 4He an
energy level for the first (0^+;0) excited state in remarkable agreement with
the experimental value. The corresponding 6Li binding energy is about 70% of
the experimental value, consistent with the expansion parameter of the EFT.
