f‐Level Green's Function for an Intermediate‐Valence Impurity

Mori's technique is used to rederive the f‐level Green's function of a mixed valence impurity with two arbitrary ionic configurations. The result agrees with the Brillouin‐Wigner resummation of Goldstone diagrams by Grewe and Keiter. The leading Brillouin‐Wigner approximation leads to an unphysical gap in the excitation spectrum of the impurity, which is closed in next leading order. The drastic change in the excitation spectrum is related to a violation of the fluctuationdissipation theorem. The consequences of a gap or continuous excitation spectrum are discussed. The advantage of the method is that only two commutators need to be evaluated in order to rederive the Brillouin‐Wigner results.