A general approach to the description of effective states of heavy atoms in molecules and solids in terms of fractional-occupancy relativistic configurations in the frames of the new Atoms-in-Compounds (AiC) concept  and two-component relativistic density functional theory is discussed. This approach is based on the analysis of molecular one-electron Kohn-Sham density matrices in the vicinity of heavy nuclei with subsequent simulation of the essential features of these entities in the calculations of the corresponding free (or confined in a spherical cavity) heavy atom. The resulting configurations are related, at least in principle, to certain measurable (spectral) properties of the compounds, for instance, chemical shifts in x-ray emission spectra. A series of applications to simple compounds of early transuranium elements (plutonium through californium) and superheavy elements (SHE) with atomic numbers 112-114 (Cn-Fl) is presented. Qualitatively different occupation patterns of effective SHE atoms and their formal lighter homologues (Hg-Pb) in the molecules of their simple binary compounds reflect the radical differences between their chemical properties. The results are compared to those obtained by the projection technique in its relativistic version  and the correlation between the effective atomic configurations and physical and chemical properties is discussed. The uselessness of non-relativistic (or scalar relativistic) configurations for understanding the essential chemistry of the mentioned SHE is underlined. In contrast, scalar relativistic model provides a reliable and transparent interpretation of peculiarities in the dependencies of molecular properties of their compounds on the actinide atomic number. Combined with the analysis of charge density and magnetization density distributions , the new approach provides an universal tool for converting the results of relativistic electronic structure calculations into traditional chemical bonding pattern.
The work was supported by the Russian Science Foundation (grant no. 14-31-00022).
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