We present a combined theoretical approach to study the nonequilibrium transport properties of nanoscale systems coupled to metallic electrodes and exhibiting strong electron-phonon interactions. We use the Keldysh Green function formalism to generalize beyond linear theory in the applied voltage an equation of motion method and an interpolative self-energy approximation previously developed in equilibrium. We analyze the specific characteristics of inelastic transport appearing in the intensity versus voltage curves and in the conductance, providing qualitative criteria for the sign of the step-like features in the conductance. Excellent overall agreement between both approaches is found for a wide range of parameters.
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