The QTAIM Perspective of Chemical Bonding

Abstract

Quantum Theory of Atoms in Molecule (QTAIM) started off as a theory of energy partitioning, interpreting a chemical bond as a result of mononuclear parts interacting with each other. These parts are topological atoms, which are defined by the topology of the electron density as subspaces with a well-defined kinetic energy of their own. Shortly after this was established, a second topological object was observed called a bond path. When taken together as a molecular graph, this object mirrored the standard Lewis diagram of covalent bonding. With a bond path is associated a bond critical point, which the community then interpreted as the signature of a chemical bond, even in cases where a Lewis diagram does not exist. Eventually, some concerns arose when curious and possibly spurious bond critical points appeared in the context of non-covalent interactions, or when traditional chemical interpretation pronounced repulsive interactions. The ultimate solution to the debate caused by these concerns is expected to come from a return to energy partitioning, but then beyond the original kinetic energy analysis and the atomic virial theorem that QTAIM started with. A body of work, under the name of Interacting Quantum Atoms (IQA), has already provided a modern reinterpretation of the bond critical point. In particular, the exchange-correlation energy between topological atoms A and B, denoted Vxc(A, B), proves to be a reliable and physically well-founded measure of covalent-like interaction energy. More importantly, it is an instrument to have a molecular graph, which is the QTAIM-equivalent of a bonding diagram, draw itself.