  The quantum transport through nanoscale junctions is governed by the charging
energy $U$ of the device. We employ the recently developed scattering-states
numerical renormalization group approach to open quantum systems to study
nonequilibrium Green functions and current-voltage characteristics of such
junctions for small and intermediate values of $U$. The reliability of the
approach is established by the excellent agreement with diagrammatic
Kadanoff-Baym-Keldysh results at small values of the $U$. We demonstrate the
limits of the diagrammatic approaches at intermediate Coulomb repulsion. These
approaches predict two different low-energy scale for magnetic and charge
fluctuations in zero bias while the numerical renormalization group approach
correctly yields only one single, universal scale. At large voltages and
intermediate values of the Coulomb repulsion the self-consistent second Born as
well as the GW approximation reproduce the SNRG spectral functions quite well
for a symmetric junctions, while for the asymmetric model the voltage-dependent
redistribution of spectral weight differs significantly. The second-order
perturbation theory does not capture the correct single-particle dynamics at
large bias and violates current conservation for asymmetric junctions.
