  The interaction between electronic and vibrational degrees of freedom in
single-molecule junctions may result from the dependence of the electronic
energies or the electronic states of the molecular bridge on the nuclear
displacement. The latter mechanism leads to a direct coupling between different
electronic states and is referred to as nonadiabatic electronic-vibrational
coupling. Employing a perturbative nonequilibrium Green's function approach, we
study the influence of nonadiabatic electronic-vibrational coupling in model
molecular junctions. Thereby we distinguish between systems with well separated
and quasi-degenerate electronic levels. The results show that the nonadiabatic
electronic-vibrational interaction can have a significant influence on the
transport properties. The underlying mechanisms, in particular the difference
between nonadiabatic and adiabatic electronic-vibrational coupling, are
analyzed in some detail.
