  Recent evidence for the top mass in the region of 160 $GeV$ for the first
time provides an opportunity to use the full power of relativistic quantum
field theoretical methods, available also for weakly bound systems. Because of
the large decay width $\G$ of the top quark individual energy-levels in
"toponium" will be unobservable. However, the potential for the $t\bar{t}$
system, based on a systematic expansion in powers of the strong coupling
constant $\a_s$ can be rigorously derived from QCD and plays a central role in
the threshold region. It is essential that the neglect of nonperturbative
(confining) effects is fully justified here for the first time to a large
accuracy, also just {\it because} of the large $\G$. The different
contributions to that potential are computed from real level corrections near
the bound state poles of the $t\bar{t}$-system which for $\G \ne 0$ move into
the unphysical sheet of the complex energy plane. Thus, in order to obtain the
different contributions to that potential we may use the level corrections at
that (complex) pole. Within the relevant level shifts we especially emphasize
the corrections of order $O(\a_s^4 m_t)$ and numerically comparable ones to
that order also from electroweak interactions which may become important as
well.
