Exponential Relationships Capturing Atomistic Short-Range Repulsion from the Interacting Quantum Atoms (IQA) Method.

A topological atom is a quantum object with a well-defined intra-atomic energy, which includes kinetic energy, Coulomb energy, and exchange energy. In the context of intermolecular interactions, this intra-atomic energy is calculated from supermolecular wave functions, by using the topological partitioning. This partitioning is parameter-free and invokes only the electron density to obtain the topological atoms. In this work, no perturbation theory is used; instead, a single wave function describes the behavior of all van der Waals complexes studied. As the monomers approach each other, frontier atoms deform, which can be monitored through a change in their shape and volume. Here we show that the corresponding atomic deformation energy is very well described by an exponential function, which matches the well-known Buckingham repulsive potential. Moreover, we recover a combination rule that enables the interatomic repulsion energy between topological atoms A and B to be expressed as a function of the interatomic repulsion energy between A and A on one hand, and between B and B on the other hand. As a result a link is established between quantum topological atomic energies and classical well-known interatomic repulsive potentials.