  The calculation on the lattice of cross--sections, form--factors and decay
rates associated to phenomenologically relevant physical processes is
complicated by the spatial momenta quantization rule arising from the
introduction of limited box sizes in numerical simulations. A method to
overcome this problem, based on the adoption of two distinct boundary
conditions for two fermions species on a finite lattice, is here discussed and
numerical results supporting the physical significance of this procedure are
shown.
