Calculations of nuclear magnetic shielding constants based on the exact two-component relativistic method.

From the matrix representation of the modified Dirac equation based on the restricted magnetically balanced gauge-including atomic orbital (RMB-GIAO) basis, previously one of the authors (Yoshizawa) and co-workers derived the two-component normalized elimination of the small component (2c-NESC) formulas for 2c relativistic calculations of nuclear magnetic resonance (NMR) shielding tensors. In the present study, at the Hartree-Fock (HF) level, we numerically confirm that for several molecules the RMB-GIAO-based 2c-NESC method provides gauge-origin independent NMR shielding values. Moreover, we investigate the accuracy of the 2c-NESC method by comparison with the 4c relativistic NMR calculations at the HF level. For noble gas dimers and Hg compounds, it is shown that the 2c-NESC method reproduces the 4c relativistic NMR shielding constants within errors of 0.12%-0.31% of the 4c relativistic values and yields chemical shifts sufficiently close to the 4c relativistic results. Also, we discuss the basis set convergence of NMR shielding constants calculated with the 2c-NESC and 4c relativistic methods.