Relativistic Effects on Bond Critical Points and Basin Population on Heavy-Atom-Containing Molecules With the Modified Dirac Equation

In this work, the topology of the scalar field at four-component (4c) level, including bond critical points (BCPs) and basin populations, is studied for SnH_4-nI_n (n = 0, …, 4) and UF₆ molecular systems. The basin population of the 4c scalar fields is calculated for the first time using the near-grid method combined with the Lebedev and Chebyshev quadratures, which improves the efficiency of the methodology applied in our previous work [A. D. Zapata-Escobar, A. F. Maldonado. Definitions of atoms in molecules with the modified Dirac equations. J. Chem. Phys. 164, 024114 (2026)]. The scalar fields used are the Dirac electron density ( ${\rho }^D$ ) and the modified Dirac scalar field ( ${\varphi }^{\mathrm{MD}}$ ). In Sn-containing molecules, it was found that the Sn-BCP distance in the Sn-H bond increases when relativistic effects are included, and there is also an increase in the electron density at the BCPs, indicating that the covalent character of the Sn-H bond is enhanced. In the case of the Sn-I bond, the Sn-BCP distance also increases with relativistic effects, but a decrease in the covalent character is observed as the electron density decreases. For the UF₆ molecule, relativistic effects move the BCPs closer to the uranium (central) atom and far away to the fluorine atoms, and this behavior is opposite to that obtained for the tin (central) atom in Sn-containing systems. Furthermore, relativistic effects produce changes in the electronic populations, with respect to the nonrelativistic values, exceeding 10% when heavy atoms are involved. In summary, it is observed that the basin associated with the heaviest atom in the studied systems is contracted by relativistic effects.