Concepts and vocabulary

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This section aims to report questions and answers on the concept and the vocabulary used by theoretical chemists involved in this field. Chemical concepts belonging to bonding theories have often vague definitions and the the words naming them are polysemous. These facts not only hamper the communication but could also lead to misunderstanding. The terminology in Chemistry is regulated by the International Union of Pure and Applied Chemistry (IUPAC) which publishes glossaries such the Compendium of Chemical Terminology and the Glossary of terms used in theoretical organic chemistry. Unfortunatly IUPAC has not a division dedicated to theoretical and computational chemistry and therefore a significant part of our vocabulary is missing in the glossaries. Moreover, the existing definitions are not always workable and sometimes they have not received a large enough consensus among the community. As an example, the covalent bond definition as: A region of relatively high electron density between nuclei which arises at least partly from sharing of electrons and give rise to an attractive force and characteristic internuclear distance, is not very helpful because relying on the model/method dependent concept of electron sharing.

• Question 1. Are there covalent intermolecular interactions? In other words, is the "non-covalent" prefix overused and meaningless?[Angel Martin Pendas]

[Bernard Silvi]

• Question 2. What are the physical mechanism of covalent and non covalent interactions and what is exactly the difference between the two terms? [Gernot Frenking]

• Question 3. Is it possible for dispersion interactions to exhibit repulsive behavior? (Several studies have reported repulsive dispersion interactions.[Eduard Matito]

• Question 4. From a physical perspective, dispersion interactions are ubiquitous. However, as chemists, we only engage in discussions about dispersion interactions when they hold relevance from a bonding perspective. Where should we draw this boundary? [Eduard Matito]

• Question 5. How is the intermolecular regime differentiated from the intramolecular one? Is this connected to the methodologies (e.g. MOs or SAPT) used to study a system?[Angel Martin Pendas]

• Question 6. The classification of intermolecular interactions is intimately coupled to perturbation theory, thus breaking when the latter ceases to be valid. In the continuum of interactions, how do the terms used, e.g. electrostatic, exchange-repulsion, polarization vs Pauli repulsion, orbital interaction, etc map onto each other?[Angel Martin Pendas]

• Question 7. Does the σ-hole model of halogen-, tetrel- or pnictogen-bonding stand still out of perturbation theory?[Angel Martin Pendas]

• Question 8. The term bond is defined by IUPAC as follows: "Bond: There is a chemical bond between two atoms or groups of atoms in the case that the forces acting between them are such as to lead to the formation of an aggregate with sufficient stability to make it convenient for the chemist to consider it as an independent 'molecular species'". To be able to define covalent and non-covalent bonds, shouldn't we also amend this definition to introduce quantitative criteria, experimentally and/or theoretically determined, even if it means having several "scales" based on different measurables? [Emilie-Laure Zins]