Two-pion exchange and strong form-factors in covariant field theories
  In this work improvements to the application of the Gross equation to nuclear systems are tested. In particular we evaluate the two pion exchange diagrams, including the crossed-box diagram, using models developed within the spectator-on-mass-shell covariant formalism. We found that the form factors used in these models induce spurious contributions that violate the unitary cut requirement. We tested then some alternative form-factors in order to preserve the unitarity condition. With this new choice, the difference between the exact and the spectator-on-mass-shell amplitudes is of the order of the one boson scalar exchange, supporting the idea that this difference may be parameterized by this type of terms. 
G. Ramalho (1), A. Arriaga (1) and M. T. Pe\~na (1,2) ((1) CFNUL   Lisbon, (2) IST Lisbon)