Magnetic shielding constants calculated by the infinite-order Douglas-Kroll-Hess method with electron-electron relativistic corrections.

We presented a two-component relativistic quantum-chemical theory for magnetic shielding constants, which is based on the infinite-order Douglas-Kroll (IODK) transformation. Two-electron relativistic corrections were also generated using the IODK transformation, although negligibly small terms were discarded. The use of small-component basis functions was completely excluded from the present theory. We examined the combination of the levels of relativistic one- and two-electron terms and magnetic interaction terms using the first-order Foldy-Wouthuysen (FW1), the second-order Douglas-Kroll (DK2), and the infinite-order Douglas-Kroll (IODK) transformations, as well as the lowest-order (c(-2)) Breit-Pauli approximation. We calculated the magnetic shielding constants of several closed-shell atoms using the FW1, DK2, IODK, and Breit-Pauli Hamiltonians. The IODK Hamiltonian reproduced well the results calculated by the four-component Dirac-Fock-Coulomb theory: The maximum deviation is only about 2.2%. We found that the accuracy of the magnetic shielding constants is strongly affected by the relativistic treatments of one-electron magnetic interaction, while the effect of the two-component two-electron relativistic corrections is relatively small. We also discussed the picture change effect on magnetic operators.