  We analyze tunneling-induced quantum fluctuations in a single-level quantum
dot with arbitrarily strong onsite Coulomb interaction, generating cotunneling
processes and renormalizing system parameters. For a perturbative analysis of
these quantum fluctuations, we remove off-shell parts of the Hamiltonian via a
canonical transformation. We find that the tunnel couplings for the transitions
connecting empty and single occupation and connecting single and double
occupation of the dot renormalize with the same magnitude but with opposite
signs. This has an important impact on the shape of the renormalization
extracted for example from the conductance. Finally, we verify the
compatibility of our results with a systematic second-order perturbation
expansion of the linear conductance performed within a diagrammatic real-time
approach.
