  The anomalous low energy behaviour observed in metals with strong electron
correlation, such as in the heavy fermion materials, is believed to arise from
the scattering of the itinerant electrons with low energy spin fluctuations. In
systems with magnetic impurities this scattering leads to the Kondo effect and
a low energy renormalized energy scale, the Kondo temperature $T_{\rm K}$. It
has been generally assumed that these low energy scales can only be accessed by
a non-perturbative approach due to the strength of the local inter-electron
interactions. Here we show that it is possible to circumvent this difficulty by
first suppressing the spin fluctuations with a large magnetic field. As a first
step field-dependent renormalized parameters are calculated using standard
perturbation theory. A renormalized perturbation theory is then used to
calculate the renormalized parameters for a reduced magnetic field strength.
The process can be repeated and the flow of the renormalized parameters
continued to zero magnetic field. We illustrate the viability of this approach
for the single impurity Anderson model. The results for the renormalized
parameters, which flow as a function of magnetic field, can be checked with
those from numerical renormalization group and Bethe ansatz calculations.
