  Transport through nanosystems is treated within the second order von Neumann
approach. This approach bridges the gap between rate equations which neglect
level broadening and cotunneling, and the transmission formalism, which is
essentially based on the single-particle picture thereby treating many-particle
interactions on an approximate level. Here we provide an alternative
presentation of the method in order to clarify the underlying structure.
Furthermore we apply it to the problem of cotunneling. It is shown that both
elastic and inelastic cotunneling can be described quantitatively, while the
transmission approach with a mean-field treatment of the interaction provides
an artificial bistability.
