The Diamagnetic and Paramagnetic Contributions to Shielding

The paramagnetic term arises from the mixing of certain excited states with the ground electronic state in the presence of a magnetic field. Because of the nature of the 

For a free atom, the natural choice of gauge origin is the nucleus in question, and the shielding then becomes equal to cr in Ramsey s formula, which simplifies to 

Values of a have been calculated for neutral free atoms for Z = 2 up to 86 from nonrelativistic ground state atomic functions. Relativistic calculations show that these values are too small for neutral atoms with high atomic number or for highly charged positive ions.  

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