  The expected large mass $m_t$ of the top quark provides for the first time a
chance to discuss the bound-states of the corresponding quantum system as the
nonabelian generalization of positronium, using the full power of relativistic
quantum field theoretic methods which are available for weakly bound systems.
Thus our approach differs in principle from the one used in the vast
phenomenological literature on quarkonium potentials. We emphasize especially
the corrections of energy levels which are of order $\a^4 m_t$ or numerically
comparable to that order, and which have no counterpart in the 'relativistic'
corrections of QED. In contrast to previous computations we give analytic
expressions for all contributions considered in our present work, hopefully
preparing the ground for further similar calculations.
