Performance of GESC and LRESC models for heavy-atom nuclear magnetic shieldings

Some models have been developed recently to calculate and analyze the electronic origin of the nuclear magnetic shielding tensor. We present here the most recent results of calculations performed with the Geometric Elimination of the Small Component (GESC) model, which represents a partial improvement of the Linear Response Elimination of the Small Component (LRESC) model, particularly concerning diamagnetic contributions. We have found that the accuracy of the diamagnetic contributions obtained with the GESC model is higher than the one obtained with the LRESC model for any molecular system. The difference between the LRESC (σdLRESC) and the four-component (σpp) methods is mainly due to the Fermi contact mechanism (σFC,LRESC). In the case of the GESC model, the contribution of such electronic mechanism is lowered by a factor of 5/7 with respect to σpp. Furthermore, the next important mechanism that contributes to the differences between σdLRESC and σpp is known as DiaK (σDiaK), being its values close to half of that due to σFC,LRESC. We analyze here the electronic mechanisms involved in the NMR shielding of halogen atoms of the following family of compounds: HX, IX, and AtX, where X = H, F, Cl, Br, I, At, and the shielding of the central atoms in the following family of molecules: Sn4-iXi and PbH4-iXi with i = 1-4 and X = H, F, Cl, Br, I. The calculations were performed at the LRESC-HF/DFT and GESC-HF/DFT levels of theory together with four-component DHF.