  We discuss how to apply many-body methods to correlated nanoscopic systems,
and provide general criteria of validity for a treatment at the dynamical mean
field theory (DMFT) approximation level, in which local correlations are taken
into account, while non-local ones are neglected. In this respect, we consider
one of the most difficult cases for DMFT, namely for a quasi-one-dimensional
molecule such as a benzene ring. The comparison against a numerically exact
solution shows that non-local spatial correlations are relevant only in the
limit of weak coupling between the molecule and the metallic leads and of low
inter-atomic connectivity, otherwise DMFT provides a quantitative description
of the system. As an application we investigate the role of correlations on
electronic transport in quantum junctions, and we show that a local
Mott-Hubbard crossover is a robust phenomenon in sharp nanoscopic contacts.
