The Schrieffer–Wolff transformation is a unitary transformation enabling one to obtain effective low energy physics from a given interacting and non-interacting Hamiltonians. An important application of Schrieffer–Wolff method is the derivation of Kondo model from single impurity Anderson model, as first noticed by J.R. Schrieffer and P.A. Wolff.
References
Schrieffer–Wolff transformation Wikipedia(Text) CC BY-SA