Convergence of DKH Energies and Variational Stability

It has already been stated in chapter 11 that an expansion in the formal parameter 1 /c is invalid because of the linear momentum associated with this formal expansion parameter. The only other remaining parameter V is the basis for 

While DKH (energy) expectation values converge smoothly to the Dirac reference, the DKH eigenfunctions (beyond first order) feature a weakly singular behavior at a point-like atomic nucleus [639,640] as the Dirac spinors do (the latter we have discussed in chapter 6). Only for a finite nucleus, the DKH eigenfunctions will feature a zero slope at the nucleus center. 

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