What is Anderson orthogonality theorem?

The Anderson orthogonality theorem is a theorem in physics by the physicist P. W. Anderson.

It relates to the introduction of a magnetic impurity in a metal. When a magnetic impurity is introduced into a metal, the conduction electrons will tend to screen the potential V {\displaystyle V} that the impurity creates. The N-electron ground state for the system when V = 0 {\displaystyle V=0} , which corresponds to the absence of the impurity and V ≠ 0 {\displaystyle V\neq 0} , which corresponds to the introduction of the impurity are orthogonal in the thermodynamic limit N → ∞ {\displaystyle N\to \infty }.