Ab initio descriptions of nuclear reactions appear to be more difficult as the reacting systems get larger. In recent researches, efforts have been made to calculate scattering amplitudes, with input of the energy levels of many-nucleon systems trapped by a harmonic-oscillator potential. By constructing an energy-dependent effective field theory, I'll introduce how to connect the energy levels to scattering amplitudes. I'll also show a method for disentangling the trapping force at short distances and the intrinsic interaction between the particles.