Energy and Real Space Characteristics of Non-Covalent Interactions Across the Periodic Table

Weak hydrogen bonds, pnictogen bonds, and halogen bonds are examined through comprehensive energy decomposition and electron density topology analysis to establish unified characterisation criteria for non-covalent interactions. Energy decomposition analysis reveals that these interactions exist along a continuum defined by the relative contributions of electrostatic, orbital, and dispersive components, with linear hydrogen bonds exhibiting predominantly electrostatic character, $\pi$ -hydrogen bonds showing balanced orbital-dispersion contributions, and pnictogen bonds demonstrating dispersion-dominated behaviour similar to lighter halogen systems. Non-covalent interaction (NCI) analysis provides a unifying framework where interaction character correlates systematically with spatial distribution: dispersive interactions generate extended, diffuse NCI volumes whilst electrostatic interactions produce compact, localised regions. The charge-to-volume ratio $\frac{q_{\mathrm{NCI}}}{V_{\mathrm{NCI}}}$ emerges as a quantitative descriptor of this localisation continuum, with eigenvalue analysis through $\delta ={\lambda }_2/{\lambda }_1$ providing complementary directional information. This electron density-based classification transcends traditional interaction nomenclature, offering systematic prediction of both structural stability and dynamic behaviour across diverse non-covalent systems.