On the essential spectrum of the Jansen-Hess operator for two-electron   ions
  Based on the HVZ theorem and dilation analyticity of the pseudorelativistic no-pair Jansen-Hess operator, it is shown that for subcritical potential strength (Z < 90) the singular continuous spectrum is absent. The bound is slightly higher (Z < 102) for the Brown-Ravenhall operator whose eigenvalues are, by the virial theorem, confined to <2m if Z<50.math/m 
D.H.Jakubassa-Amundsen