KP-approach for non-symmetric short-range defects: resonant states and   alloy bandstructure
  The short-range defect with reduced symmetry is studied in the framework of KP-approach taking into account a matrix structure of potential energy in the equations for envelope functions. The case of the narrow-gap semiconductor, with defects which are non-symmetric along the [001], [110], or [111] directions, is considered. Resonant state at a single defect is analyzed within the Koster-Slater approximation. The bandstructure modification of the alloy, formed by non-symmetric impurities, is discussed and a generalized virtual crystal approximation is introduced. 
F.T. Vasko