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

## TL;DR

This paper explores how different types of non-covalent bonds behave across the periodic table using energy and electron density analysis.

## Contribution

The study introduces a unified framework using energy decomposition and electron density to classify non-covalent interactions.

## Key findings

- Hydrogen bonds are mostly electrostatic, while pnictogen bonds are dispersion-dominated.
- NCI analysis links electrostatic and dispersive contributions through a charge-to-volume ratio.
- The framework allows systematic prediction of structural stability and dynamic behavior in non-covalent systems.

## Abstract

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, π‐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 qNCIVNCI emerges as a quantitative descriptor of this localisation continuum, with eigenvalue analysis through δ=λ2/λ1 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.

Energy decomposition and electron density analyses reveal that hydrogen, pnictogen, and halogen bonds belong to a single continuum of non‐covalent interactions. The NCI charge‐to‐volume ratio quantitatively links electrostatic and dispersive contributions, providing a unified, density‐based description of interaction character across bonding families.