  Considering model Hamiltonians that respect the symmetry properties of the
pnictides, it is argued that pairing interactions that couple electrons at
different orbitals with an orbital-dependent pairing strength inevitably lead
to interband pairing matrix elements, at least in some regions of the Brillouin
zone. Such interband pairing has not been considered of relevance in
multiorbital systems in previous investigations. It is also observed that if,
instead, a purely intraband pairing interaction is postulated, this requires
that the pairing operator has the form $\Delta^+(k)=f(k) \sum_{\alpha}
d^+_{k,\alpha,\uparrow}d^+_{-k, \alpha,\downarrow}$ where $\alpha$ labels the
orbitals considered in the model and f(k) arises from the spatial location of
the coupled electrons or holes. This means that the gaps at two different Fermi
surfaces involving momenta $k_F$ and $k'_F$ can only differ by the ratio
$f(k_F)/ f(k'_F)$ and that electrons in different orbitals must be subject to
the same pairing attraction, thus requiring fine tuning. These results suggest
that previously neglected interband pairing tendencies could actually be of
relevance in a microscopic description of the pairing mechanism in the
pnictides.

Superconductivity;Strongly Correlated Electrons