Optical Theorem and Electronic States in Disordered Transition Metals

In disordered systems, apparently reasonable decoupling procedures can lead to negative densities of electronic states near resonant $d$ bands. By applying the optical theorem to the multiple-scattering series we can determine a prior whether a given decoupling will necessarily yield positive results. Since none of the extensions of the coherent-potential approximation to amorphous systems is found to meet this requirement, we propose a new scheme which is both self-consistent and guaranteed to give a positive spectrum.