  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.
