QCD Box Graphs and the Quark-Antiquark Potential
  The $t\bar{t}$ system allows a truly perturbative treatment of the potential. Completing previous computations, we calculate the contributions of QCD box graph corrections, which make the ``relativistic'' $O(\a^4)$ corrections in the non-Abelian case differ from the well known corrections of the same order in QED. 
W. Kummer, W. Moedritsch and A. Vairo