Modified Dirac equation regular

Finally, we mention the regular modified Dirac equation of Sadlej and Snijders (1994), which is exact and follows the lines of chapter 15. The regular transformation of the small component. 

This equation is exact just as the modified Dirac equation of chapter 15 is exact. It can also be separated into spin-free and spin-dependent terms, but now the separation must be done in both the Hamiltonian and the metric. Visscher and van Lenthe (1999) have shown that the spin separation gives different results for the two modified equations, and therefore the spin separation is not unique. This regular modified Dirac equation can be used in renormalization perturbation theory, with ZORA as the zeroth-order Hamiltonian. 

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