Chapter 4 - New developments in the Interacting Quantum Atoms (IQA) approach

Abstract

The Interacting Quantum Atoms (IQA) approach, formulated within the framework of Quantum Chemical Topology (QCT), has become one of the most useful tools for the rigorous study of Chemistry. In IQA, the total wavefunction energy is broken down in one-body and two-body pairwise contributions which emerge from well-defined quantum mechanical operators, allowing chemists to recover nearly the entire spectrum of chemical intuitive terms form the former. Indeed, hitherto, this and similar methodologies have been successfully employed to explore different fields within the chemical world, including the intriguing domain of molecular reactivity. This chapter aims to provide the reader a brief overview of how QCT in general, and IQA in particular, provides an ideal framework to derive a wide range of tools targeted at shining light on to multiple chemical phenomena. Special emphasis is given to the field of chemical reactivity. The chapter covers both theoretical and practical aspects of this topic. First, a brief introduction of energy decomposition methods is provided, followed by the IQA method itself, for which a more in-detail picture is given, and particularly within the context of Density Functional Theory (DFT). Then, the reader will delve deeper into the chemical insights that can be obtained with the IQA prism. Finally, and as a proof of concept, the very last section shows the plethora of intuitive results afforded through IQA in a prototypical reaction process: the textbook example of the 1,3-dipolar cycloaddition between ethylene and acetonitrile oxide molecules.