On Scalar and Vector Potentials for the Nonlinear Electromagnetic Forces
  The potential concept that is successful in classical electrodynamics should also be applicable to the nonlinear electromagnetic forces acting on matter. The obvious method of determining these potentials should be provided by Helmholtz's theorem. It is found, however, that the theorem fails in most practical instances. Other methods to find the potentials - as pursued in plasma physics - are examined and found to yield functions which depend on the chosen coordinate system. Thus they cannot be considered as invariant potentials from which physical forces may be derived. Practical consequences of these mathematical findings are discussed. 
Wolfgang Engelhardt