several iterms are found in the literatures on the calculation works
of TM system,such as:

scalar-relativistic(for example,ZORA,DKH)
quasi-relativistic(for example,pseudopotential)
two-component relativistic
one-component realtivistic

what's the relation and difference among them ?
and what levels of approximation are they at relative to
full-relativistic(also as four-component relativistic),respectively ?

thanks!

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