  We use the scattering states numerical renormalization group (SNRG) approach
to calculate the current $I(V)$ through a single molecular level coupled to a
local molecular phonon. The suppression of $I$ for asymmetric junctions with
increasing electron-phonon coupling, the hallmark of the Franck-Condon
blockade, is discussed. We compare the SNRG currents with recently published
data obtained by an iterative summation of path integrals approach (ISPI). Our
results excellently agree with the ISPI currents for small and intermediate
voltages. In the linear response regime $I(V)$ approaches the current
calculated from the equilibrium spectral function. We also present the
temperature and voltage evolution of the non-equilibrium spectral functions for
a particle-hole asymmetric junction with symmetric coupling to the lead.
