Spin–orbit corrections to NMR shielding constants from density functional theory. How important are the two-electron terms?

A recently developed density functional theory (DFT) ansatz for the calculation of one-electron spin–orbit (SO) corrections to NMR chemical shifts has been extended to include two-electron SO operators. The performance of different approximations is evaluated in calculations of 1H and 13C nuclear shieldings in the hydrogen halides, the halomethanes, and in iodobenzene. Two-electron contributions are significant relative to the one-electron contributions for fluorine substituents (ca. 30–35%) but decrease for the heavier halogens (ca. 6–7% for iodine). A mean-field approximation to the two-electron SO integrals performs excellently, with negligible deviations from the `exactʹ results, thus opening the way to calculations on large systems.