  We discuss the dynamics of a spin coupled to a damped harmonic oscillator.
This system can be mapped to a spin-boson model with a structured bath, i.e.
the spectral function of the bath has a resonance peak. We diagonalize the
model by means of infinitesimal unitary transformations (flow equations),
thereby decoupling the small quantum system from its environment, and calculate
spin-spin correlation functions.
