The charge transfer spectral properties of group IV A tetrahalides (where M=Ti, Zr or Hf and X=F, Cl, Br or I) have been calculated using the Ms-Xα method and compared with experimental values. The order of ligand-to-metal charge transfer energies corresponds to the one-electron oxidation potential of X→X+e⁻(F⁻/F, Cl⁻/Cl, Br⁻/Br and I⁻/I). The dependence of the ligand-to-metal charge transfer energies on the optical electronegativity of the ligand has been demonstrated. The nature of bonding of the group IVA tetrahalides has been analysed using the MS-Xα method.

Theoretical calculations have been performed on Si₂C using density functional theories (DFT) and conventionalab initio method employing 6-311G* basis set. Present calculations indicate that some of the DFT methods (B3LYP, B3P86, BHandH, BHandHLYP) employed in this study cannot predict the observed bent structure in the case of Si₂C cluster whereas, MP2, SVWN5 and B3PW91 calculations predict the bent structure, as observed by experiment.

Systematic investigation has been carried out to calculate critical points, charge density,ρ(r_c), and Laplacian of charge density, ∇²ρ(r_c), for weak complexes in the framework of MP2 and density functional theory (DFT). This study helps in assigning structures to van der Waals complexes such as Rg-HF, Rg-HCl, Rg-Li⁺ and Rg-Mg⁺, where Rg=He, Ne and Ar. The presence of (3, −1) critical points (bond critical points) in the complexes and the typical values ofρ(r) and ∇²ρ(r) at critical points indicate that closed-shell type of interaction is present in the molecules. The possible difference between the two types of bonding, rare gas-hydrogen bond interaction (Rg-H-X) and rare gas-halogen bond interaction (Rg-X-H), (where X=F and Cl) has also been analysed by computing the properties of charge density and Laplacian of charge density at the bond critical points.

In this perspective article, the basic theory and applications of the “Quantum Theory of Atoms in Molecules” have been presented with examples from different categories of weak and hydrogen bonded molecular systems.

In this article, the self-assembly of cyanuric acid (CA) molecules into nano-structures is examined. Equilibrium geometry of CA is planar and it belongs to the D_3h point group. It is shown that CA clusters form three dimensional bowls and balls. Cyclic pentamer (5-bowl) is the basic motif responsible for these non-planar geometries. It is also shown that the cyclic hexamer based clusters can be non-planar if they contain a 5-bowl. A unified criterion for the formation of bowls and balls from basic molecular building blocks emerges from this study. The role of symmetry in supramolecular self-assembly is also clearly evident from the present study.