Two-Component Electron Density Distribution

For a one-electron system, the electron density is obtained as the square of the orbital in four-component as well as in nonrelativistic theory. One might be led to believe that this also holds for transformed DKH orbitals. This, however, is not the case, and the situation is much more involved than one might at first think. For the detailed discussion of this problem we need to refer to section 15.3.5 later. 

If (pi were a real-valued function, we would have to have two different radial parts for the two components of the 2-spinor (pi in order to obtain a nodeless radial density distribution as in the four-component case. In a one-component approximation, which neglects all spin-dependent terms and which therefore does not resemble the four-component density, additional nodes may occur as in the Schrbdinger case. 

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