Halogen–Halogen Contacts in Bending Crystals: Limitations of QTAIM and NCI in Describing Sliding Energetics

The plastic bending behavior of isostructural hexachlorobenzene and hexabromobenzene crystals is governed by anisotropic intermolecular interactions, particularly halogen–halogen (Hal⋯Hal) contacts. This study critically evaluates the reliability of Quantum Theory of Atoms in Molecules (QTAIM) and Non-Covalent Interaction index analyses in capturing the energetic landscape of layer sliding along the bending (001) crystallographic plane. Energy profiles are calculated using periodic Density Functional Theory, gas-phase DFT, and the CrystalExplorer cluster-based approach. While all computational methods confirm the presence of a significant energy barrier to sliding, analysis of the interaction energy contributions computed with CrystalExplorer17 reveals that changes in the dispersion component are minimal (<2%), whereas change in the repulsive and electrostatic components dominate the sliding barrier. Notably, QTAIM-derived interaction energies for Hal⋯Hal contacts spuriously indicate that layer displacement is energetically favorable, in direct contradiction to total energy calculations. Furthermore, NCI analysis shows that steric repulsion is systematically underestimated as halogen–halogen distances decrease. These findings demonstrate that, although QTAIM and NCI methods yield valuable local geometric descriptors, they are insufficient and potentially misleading for quantifying the total energy balance underpinning the mechanical response of these halogen-rich plastic crystals.