Few-Body Nature of Kondo Correlated Ground States

In the previous part, it was discussed that despite the apparent simplicity of quantum impurity models, they hide non trivial physics and their solution is in general a hard task. Indeed, the many-body description of these problems forces one to work in the exponentially large Fock space where every state, or Slater determinant for fermionic systems that we consider, is treated equally. It could be hoped that most of this huge amount of determinants do not participate notably to the ground state characterization and it would be appealing not to consider them. For this purpose, it is desirable to find the corner of the Fock space within which only the most relevant determinants remain in the computation, changing the exponential scaling to some polynomial function of the system size.