The Still Unknown Nature of Chemical Bonding

Eric Scerri named the 2nd most influential chemist for the decade 2010-2020 gave a plenary talk at 26th Conference of the ISPhil of Chemistry in 2022 on The Nature of Chemical Bonding starting out presenting two views according to Robin Hendry:

• Structural as actual "bonds".
• Energetic as "bonding". 

Scerri continues recalling Coulson's words (1955):

• A bond does not exist: no-one has ever seen it, no-one ever can. It is a figment of our own imagination.

Scerri adds a statement by his colleague Seifert from an email conversation:

• Neither the structural nor the energetic provide "suitable characterization of chemical bonding". 

This is not a good start in 2022 in view of Gilbert Lewis clarification in 1916: 

• In the mind of the organic chemist the chemical bond is no mere abstraction; it is a definite physical reality. a something that binds atom to atom.  

Scerri then proceeds to scrutinise of how chemical bonding is described in text books of chemistry concluding that there is no satisfactory explanation of in particular the covalent bond somehow emerging from "sharing of electrons" as the dominating theme (together with the ionic bond as simply Coulomb attraction between ions). 

Scerri struggles to make sense of the textbook explanations following the very natural idea that somehow a bond can form if electrons can find a configuration close to kernels with decreasing potential energy which is not balanced by increase of kinetic energy supposedly emerging from two conflicting actions: 

• concentration of electron charge distribution decreasing potential energy
• delocalisation of electrons charge distribution avoiding increase of kinetic energy.   

But Scerri stops there: The Nature of Chemical Bonding is still a mystery, since it is the title of hus talk in 2022. But it must have a nature in physical terms, it cannot just be imagination. The World consists of molecules as atoms held together by chemical bonds.

It seems to me that Scerri is saying that only modest progress has been made over more than 100 years, despite the giant progress of modern physics in the form of quantum mechanics supposedly describing all of the physics of atoms and molecules including chemical bonding. How can this be? How is it possible that the nature of chemical bonding is still basically a mystery?

Here RealQM comes in offering a physical explanation how electrons can concentrate between atomic kernels to form a "glue" from decrease of potential energy without increase of kinetic energy. The secret is revealed in this plot as print-out of this code from a previous post from a collection of posts on H2:

We see two H kernels in black and a cross-cut along a line through the kernels of two electron wave functions with non-overlapping supports meeting at a plane orthogonal to the line midway between the kernels (as a free boundary) in yellow with plots of characteristic functions of the supports in black.

Computations are 3d on a fixed mesh with 1 million mesh points with mesh sise 0.1 in atomic units. The code is fully explicit consisting of 3 lines for update pf wave functions, potentials and free boundary. The computed energy  (-1.146) is in fair agreement with observation (-1.17) with further improvement under additional iterations. 

The essence is that in RealQM electron wave functions can meet with non-zero joint values at a free boundary which makes concentration of electron charge between the kernels possible without increase of kinetic energy. 

RealQM offers a new theoretical explanation of chemical bonding which agrees with previous intuition without theory.  Here are posts on Real Quantum Chemistry.