  The relativistic fermion-antifermion bound state vector potential of
constraint theory is calculated, in perturbation theory, by means of the
Lippmann-Schwinger type equation that relates it to the scattering amplitude.
Leading contributions of n-photon exchange diagrams are calculated in an
approximation scheme that adapts eikonal approximation to the bound state
problem. They produce terms proportional, in three-dimensional x-space, to
$(\alpha/r)^n$. The series of leading contributions is summed.
