Interacting Quantum Atoms (IQA)

Overview

The Interacting Quantum Atoms (IQA) methodology, developed in our group, partitions the total electronic energy of a molecule rigorously into intra-atomic self-energies and pairwise inter-atomic interaction energies. Each inter-atomic interaction is further decomposed into classical electrostatics and quantum exchange–correlation contributions.

Because IQA is grounded in the QTAIM atomic partition, it is invariant to orbital transformations and does not depend on any reference state or arbitrary choices of localisation. This makes IQA particularly valuable for:

• Comparing bonding across different molecular environments and bond types.
• Tracking energy changes along reaction coordinates and conformational changes.
• Establishing connections between bond strength and electron delocalisation.

IQA and Bond Orders

A central result of IQA analysis is the relationship between the inter-atomic exchange–correlation energy V_XC(A,B) and the classical bond order between atoms A and B. The delocalization index δ(A,B)—the number of electron pairs shared between basins—provides an orbital-invariant bond order.

Applications

IQA has been applied in our group to:

• Hydrogen and halogen bonds
• Metal–ligand bonding in transition metal complexes
• π-stacking and non-covalent interactions
• Reaction mechanisms and transition state analysis