The correlation theory of the chemical bond

TL;DR

This paper introduces a comprehensive multiorbital correlation theory to better understand chemical bonds, providing new measures and algorithms to analyze complex multiorbital interactions and clarify bonding structures.

Contribution

It presents the first true multiorbital correlation framework, including measures, clustering methods, and algorithms, advancing the analysis of chemical bonds beyond two-orbital correlations.

Findings

Identifies cases where multiorbital correlations challenge traditional bonding pictures.

Provides quantitative tools to assess the adequacy of bonding models.

Demonstrates the theory on various molecules to reveal complex bond structures.

Abstract

The quantum mechanical description of the chemical bond is generally given in terms of delocalized bonding orbitals, or, alternatively, in terms of correlations of occupations of localised orbitals. However, in the latter case, multiorbital correlations were treated only in terms of two-orbital correlations, although the structure of multiorbital correlations is far richer; and, in the case of bonds established by more than two electrons, multiorbital correlations represent a more natural point of view. Here, for the first time, we introduce the true multiorbital correlation theory, consisting of a framework for handling the structure of multiorbital correlations, a toolbox of true multiorbital correlation measures, and the formulation of the multiorbital correlation clustering, together with an algorithm for obtaining that. These make it possible to characterise quantitatively, how well a bonding picture describes the chemical system. As proof of concept, we apply the theory for the investigation of the bond structures of several molecules. We show that the non-existence of well-defined multiorbital correlation clustering provides a reason for debated bonding picture.