Relativistic theories of NMR shielding

The theoretical picture of nuclear magnetic resonance (NMR) shielding is first discussed in depth through the body-fixed molecular Hamiltonian that treats electrons fully relativistically and nuclei quasi-relativistically. Various ways are then presented to capture the relativistic diamagnetism, which is otherwise missed by the standard four-component relativistic linear response theory of NMR shielding. With an explicit relativistic diamagnetism, the correct nonrelativistic limit of the four-component relativistic theory can be guaranteed even with a finite basis. The gauge origin problem can further be resolved by means of the idea of distributed gauge origins. The exact two-component counterpart can also be formulated in a simple way.