Further Difficulties with the Klein-Gordon Equation
  The Dirac equation is compared with the Klein-Gordon one. Unlike the Dirac case, it is proved that the Klein-Gordon equation has problems with the Hamiltonian operator of the Schroedinger picture. A special discussion of the Pauli-Weisskopf article and that of Feshbach-Villars proves that their theories of a charged Klein-Gordon particle lack a self consistent expression for this Hamiltonian. Related difficulties are pointed out. 
E. Comay