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 H-1 and C-13 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 'exact' results, thus opening the way to calculations on large systems. (C) 1998 Elsevier Science B.V. All rights reserved.