scheme [@Habilitation; @CKMworkshop]) is the most important measurement that can be obtained from the threshold scan since an uncertainty of only around 100 MeV is expected[@TTbarsim; @synopsis]. This prospect is quite robust, from the theoretical as well as from the experimental point of view, since it relies mostly on the determination of the c.m.energy where the cross section rises. Because the $t\bar t$ pair is produced predominantly in an S-wave state the rise of the cross section is quite rapid and easily measurable even in the presence of beam effects [@TTbarsim]. In addition it will also be possible to determine the strong coupling $\alpha_s$, the total top quark width $\Gamma_t$ and, if the Higgs boson is light, the top Yukawa coupling $g_{\rm tth}$. However, the latter measurements are sensitive to the form and the normalization of the line-shape. Since the observable cross section is a convolution of the theory prediction with the partly machine-dependent luminosity spectrum arising from QED effects [@TTbarsim; @Cinabro1], high demands are imposed on theoretical predictions and experimental analyses to make these measurements possible. In particular, theoretical predictions need to have a precision at the level of only a few percent.

The common theoretical tool to make 