  With an increasing complexity of nanoscopic systems and the modeling thereof,
new theoretical tools are needed for a reliable calculation of complex systems
with strong electronic correlations. To this end, we propose a new approach
based on the recently introduced dynamical vertex approximation. We demonstrate
its reliability already on the one-particle vertex (i.e., dynamical mean field
theory) level by comparison with the exact solution. Modeling a quantum point
contact with 110 atoms, we show that the contact becomes insulating already
before entering the tunneling regime due to a local Mott-Hubbard transition
occurring on the atoms which form the point contact.
