We present a Car-Parrinello investigation of various active surfaces and catalytic sites in a realistic Ziegler-Natta heterogeneous system. We examine the (100), (110), and (104) surfaces of the MgCl2 support and the related binding of the possible mononuclear and dinuclear catalyst configurations. Relaxation and/or reconstruction processes affect these surfaces in varying degrees, according to the different Miller indexes. We find that TiCl4 and Ti2Cl6 species can bind as stable adducts, depending on the morphology of the surface considered. However, the activation and polymerization phases show that destabilization phenomena can affect the dinuclear species during the catalysis reaction. This provides a new insight into the ability of the different centers to give rise to the real polymerization process. Finally, we present a first attempt to address the role of a typical donor phthalate at a fully first principles level.