  The spin-boson model is studied by means of flow equations for Hamiltonians.
Our truncation scheme includes all coupling terms which are linear in the
bosonic operators. Starting with the canonical generator $\eta_c=[H_0,H]$ with
$H_0$ resembling the non-interacting bosonic bath, the flow equations exhibit a
universal attractor for the Hamiltonian flow. This allows to calculate
equilibrium correlation functions for super-Ohmic, Ohmic and sub-Ohmic baths
within a uniform framework including finite bias.
