Interactions between noble metals and rare gases have become an interesting topic over the last few years. In this work, a computational study of the open-shell (d 10 s 1 ) and closed-shell (d 10 s and d 10 s 2 ) noble metals (M ¼ Cu, Ag, and Au) with three heaviest rare gas atoms (Rg ¼ Kr, Xe, and Rn) has been performed. Potential energy curves based on ab initio [MP2, MP4, QCISD, and CCSD(T)] and DFT functionals (M06-2X and CAM-B3LYP) were obtained for ionic and neutral AuXe complexes. Dissociation energies indicate that neutral metals have the lowest and cationic metals have the highest affinities for interaction with rare gas atoms. For the same metals, there is a continuous increase in dissociation energies (D e ) from Kr to Rn. The nature of bonding and the trend of D e and equilibrium bond lengths (R e ) have been interpreted by means of quantum theory of atoms in molecules, natural bond orbital, and energy decomposition analysis.

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A new method has been developed to detect and analyze molecular π systems. The concept of bonding critical point is generalized to electronic π systems, and it is shown how a π bond can be characterized via the corresponding bond critical point (BCP) in planar molecules. In this context, charge density and its Laplacian at the BCP(π) of a strongly delocalized π system can be distinguished from that of a localized one. The presented formalism is applied to three types of nanoconductors as conjugated polyenes, which revealed the alternative pattern of the double bonds. Also, several cyclic conjugated molecules are considered to explore their π electronic structure and aromaticity.

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The kinetic energy pressure (KEP) is quantified in terms of displaced charges and their orbital representations. Two deformation density matrices are introduced to separate reorganization of the electron density due to Pauli antisymmetrization from that of orbital relaxation. The formalism is applied to interaction of carbon nanotubes with hydrogen molecule and the results confirmed that KEP has the main contribution to such interaction. Also, it was found that the contribution of KEP can be easily traced in the complex formation.