  We propose a method for the lattice QCD computation of nucleon-nucleon
low-energy interactions. It consists in simulating QCD in the background of a
''electromagnetic" field whose potential is non-vanishing, but whose field
strength is zero. By tuning the background field, phase-shifts at any (but
small) momenta can be determined by measuring the shift of the ground state
energy. Lattice sizes as small as 5 Fermi can be sufficient for the calculation
of phase shifts up to momenta of order of $m_{\pi}/2$.
