  A magnetic moment in a metal or in a quantum dot is, at low temperatures,
screened by the conduction electrons through the mechanism of the Kondo effect.
This gives rise to spin-spin correlations between the magnetic moment and the
conduction electrons, which can have a substantial spatial extension. We study
this phenomenon, the so-called Kondo cloud, by means of the density matrix
renormalization group method for the case of the single-impurity Anderson
model. We focus on the question whether the Kondo screening length, typically
assumed to be proportional to the inverse Kondo temperature, can be extracted
from the spin-spin correlations. For several mechanisms -- the gate potential
and a magnetic field -- which destroy the Kondo effect, we investigate the
behavior of the screening cloud induced by these perturbations.
