Contributions of IQA electron correlation in understanding the chemical bond and non-covalent interactions

Arnaldo F Silva,Paul L A Popelier,Leonardo J Duarte

doi:10.1007/s11224-020-01495-y

Abstract

The quantum topological energy partitioning method Interacting Quantum Atoms (IQA) has been applied for over a decade resulting in an enlightening analysis of a variety of systems. In the last three years we have enriched this analysis by incorporating into IQA the two-particle density matrix obtained from Møller–Plesset (MP) perturbation theory. This work led to a new computational and interpretational tool to generate atomistic electron correlation and thus topologically based dispersion energies. Such an analysis determines the effects of electron correlation within atoms and between atoms, which covers both bonded and non-bonded “through -space” atom–atom interactions within a molecule or molecular complex. A series of papers published by us and other groups shows that the behavior of electron correlation is deeply ingrained in structural chemistry. Some concepts that were shown to be connected to bond correlation are bond order, multiplicity, aromaticity, and hydrogen bonding. Moreover, the concepts of covalency and ionicity were shown not to be mutually excluding but to both contribute to the stability of polar bonds. The correlation energy is considerably easier to predict by machine learning (kriging) than other IQA terms. Regarding the nature of the hydrogen bond, correlation energy presents itself in an almost contradicting way: there is much localized correlation energy in a hydrogen bond system, but its overall effect is null due to internal cancelation. Furthermore, the QTAIM delocalization index has a connection with correlation energy. We also explore the role of electron correlation in protobranching, which provides an explanation for the extra stabilization present in branched alkanes compared to their linear counterparts. We hope to show the importance of understanding the true nature of the correlation energy as the foundation of a modern representation of dispersion forces for ab initio, DFT, and force field calculations.

Highlights

Electron correlation and energy partitioningThe London dispersion force [1]
The observations made throughout this review point to two perhaps conflicting yet forthright conclusions: (i) there is much chemical insight contained in the correlated part of the two-particle density matrix, and (ii) relatively little is known about its nature since only a few groups are investigating correlation energy by evaluating the twoparticle density matrix
The presence and influence of electron correlation is ubiquitous in a myriad of chemical systems and situations but its importance is often overshadowed by larger electronic effects and by its intricate nature, as specific software, and complicated expressions are required to accurately gage atomic electron correlation energies

Summary

Introduction

Electron correlation and energy partitioningThe London dispersion force [1] Electron correlation is typically divided into dynamical and static correlation While the latter is important for molecules where the ground state is described well only with more than one (nearly-) degenerate determinant, the former is responsible for London dispersion. In order to evaluate the Struct Chem (2020) 31:507–519 correlation energy of a system one must evaluate the twoparticle density matrix, which can be quite a daunting task. The concepts of both dispersion and electron correlation are simple and well understood but there is a gap between the theoretical basis (stabilized many years ago) and the implementation (computationally costly) of these quantities in modern molecular modeling. By the end of the review, we hope to make clear how we intend to bridge this gap and give examples of how it affects the reliability of predictions

Objectives

Findings

Discussion

Conclusion