  In a multiband Hubbard model the self-consistency relations for the two-body
bound-state bands are in the form of a nonlinear eigenvalue problem. Assuming
that the resultant eigenvectors form an orthonormal set, e.g., in the
strong-binding regime, here we reformulate their Berry curvatures and the
associated Chern numbers. As an illustration we solve the two-body problem in a
Haldane-Hubbard model with attractive onsite interactions, and analyze its
topological phase diagrams from weak to strong couplings, i.e., by keeping
track of the gap closings in between the low-lying two-body bands. The
resultant Chern numbers are consistent with the lobe structure of the phase
diagrams in the strong-coupling regime.
