SPIN-ORBIT CORRECTIONS TO NMR SHIELDING CONSTANTS FROM DENSITY-FUNCTIONAL THEORY - HOW IMPORTANT ARE THE 2-ELECTRON TERMS

A recently developed density functional theory (DFT) ansatz for the ca lculation of one-electron spin-orbit (SO) corrections to NMR chemical shifts has been extended to include two-electron SO operators. The per formance of different approximations is evaluated in calculations of H -1 and C-13 nuclear shieldings in the hydrogen halides, the halomethan es, and in iodobenzene. Two-electron contributions are significant rel ative 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 e xcellently, with negligible deviations from the 'exact' results, thus opening the way to calculations on large systems. (C) 1998 Elsevier Sc ience B.V. All rights reserved.