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 that the impurity creates. The N-electron ground state for the system when , which corresponds to the absence of the impurity and , which corresponds to the introduction of the impurity are orthogonal.