  The single impurity Kondo model at zero temperature in a magnetic field is
solved by a semi-analytical approach based on the flow equation method. The
resulting problem is shown to be equivalent to a resonant level model with a
non-constant hybridization function. This nontrivial effective hybridization
function encodes the quasiparticle interaction in the Kondo limit, while the
magnetic field enters as the impurity orbital energy. The evaluation of static
and dynamic quantities of the strong-coupling Kondo model becomes very simple
in this effective model. We present results for thermodynamic quantities and
the dynamical spin-structure factor and compare them with NRG calculations.
