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 ( r ) that the impurity creates. The N-electron ground state for the system when V ( r ) = 0 , which corresponds to the absence of the impurity and V ( r ) ≠ ≠ 0 , which corresponds to the introduction of the impurity are orthogonal in the thermodynamic limit N → → ∞ ∞ .