Smaller PNC contributions

The first correction we consider is the effect of the Breit interaction. It is taken into account by replacing the Coulomb interaction V in the basic Hamiltonian of Eq. (55) by the sura of the Coulomb and Breit interactions. 

With this replacement, the HF equations for the single-particle orbitals become 

Since the dominant contribution to the PNC amplitude is the PNC-HF contribution, it is sufficient to carry out a PNC-HF calculation including 

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