9 B. Dynamical susceptibility atomic positions, we also calculate the χ ( exp ) 0 , at the experimental position of the As atom. In Fig. 6 we plot χ ( exp ) 0 (in the basal plane) as a function of doping, and its change relative to the χ$_{0}$ calculated at the optimized position of the As atom. We see a sizeable difference between the two at zero doping: using experimental positions, the peak at the nesting vector is much more evident and broader, while the peak around Γ is depressed. We can relate this change to the differences in the electronic structure, the 3D bands crossing E$_{F}$ around the Z point become 2D, but also the radius of the FS cylinders is larger in the optimized structure making the nesting between e -like and h -like sheets less efficient. In the doped system, on the other hand, this band is completely full in both atomic configurations as a consequence of which in the doped case, the difference induced by change in atomic positions is not as significant. On comparing the static susceptibility calculated in the present work with the previous results of Mazin et al. 8 one notices that the height of the peak around the M point is significantly suppressed on inclusion of the matrix elements. This is a consequence of the fact that the states near E$_{F}$ contribute to Re χ$_{0}$ selectively, weighted by their overlap integral and not by unity. It is believed that the interacting susceptibility is a key quantity in determining the ef- fective electron-electron interactions leading to superconductivity. Our results show that the inclusion of the matrix elements influences its q dependence and in particular the rel- ative height of the peaks; Although our calculations only refer to the independent-electron susceptibility , these conclusions are likely to hold also for the interacting χ . Therefore, it is important to include the matrix elements in order to use χ for assessing the validity of models for the superconducting mechanism ( e.g. the s ± wave). An overview of the dynamical response of the system is given in Fig. 7, where we plot Im χ$_{0}$ , the imaginary part of χ$_{0}$ ( q , ω ), as a function of frequency and momentum, along the Γ − M − Γ and Γ − X − Γ lines of the Brillouin zone, for x = 0. The most prominent structure is a broad peak located at frequencies between 1 and 2 eV, which behaves in a similar manner along the Γ − M − Γ and Γ − X − Γ directions. Near the M point we also see a sharp and intense peak located at about 3 eV. At this point it is also worth mentioning that