Explicit Form of Perturbation Operators

The explicit form of the property operators is derived in two steps. First, the scalar and vector fields are determined and, second, they are inserted into the Dirac-Breit [951,952] or any other quasi-relativistic operator derived from it such as the external-field-containing DKH operator, the external-field-containing ZORA Hamiltonian or the similar Breit-Pauli Hamiltonian. The theory of NMR parameters has been derived by Ramsey [953-955] from the nonrelativistic perspective and by Pyykko from the relativistic perspective [956,957]. 

As an example of the explicit expression we give the vector potential produced by the nuclear spin of a point nucleus 

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