  An analysis of neutron and proton scattering off $^{40,48}$Ca has been
carried out. Real and imaginary potentials have been generated using the
Nuclear Structure Method (NSM) for scattering with the Gogny D1S
nucleon-nucleon effective interaction. Observables are well described by NSM
for neutron and proton elastic scattering off $^{40}$Ca and for neutron
scattering off $^{48}$Ca. For proton scattering off $^{48}$Ca, NSM yields a
lack of absorption. This discrepancy is attributed to double-charge-exchange
contribution and coupling to Gamow- Teller mode which are not included in the
present version of NSM. A recipe based on a Perey-Buck fit of NSM imaginary
potential and Lane model is proposed to overcome this issue in an approximate
way.
