We show that the mutual, through‐space compression of atomic volume experienced by approaching topological atoms causes an exponential increase in the intra‐atomic energy of those atoms, regardless of approach orientation. This insight was obtained using the modern energy partitioning method called interacting quantum atoms (IQA). This behaviour is consistent for all atoms except hydrogen, which can behave differently depending on its environment. Whilst all atoms experience charge transfer when they interact, the intra‐atomic energy of the hydrogen atom is more vulnerable to these changes than larger atoms. The difference in behaviour is found to be due to hydrogen's lack of a core of electrons, which, in heavier atoms, consistently provide repulsion when compressed. As such, hydrogen atoms do not always provide steric hindrance. In accounting for hydrogen's unusual behaviour and demonstrating the exponential character of the intra‐atomic energy in all other atoms, we provide evidence for IQA's intra‐atomic energy as a quantitative description of steric energy.

Invited for this month's cover picture is the group of Paul L. A. Popelier from Manchester Institute of Biotechnology (UK). The cover picture shows the quantum topological atoms in a configuration of the complex HF⋅⋅⋅OH2, where F is green and O is red. Read the full text of their Full Paper at 10.1002/open.201800275.

The Front Cover shows the quantum topological atoms in a configuration of the complex HF⋅⋅⋅OH2, where F is green and O is red. The background is the gradient vector field of the electron density in the plane of the complex. This work shows that the intra‐atomic energy (Eintra) of the interacting quantum atoms (IQA) approach, mainly the kinetic energy contribution, quantifies steric repulsion. This assertion is confirmed by numerous successful fits to the classical Buckingham potential, Aexp(‐Br), for a wide variety of orientations in atom⋅⋅⋅atom contact. If hardness is associated with B's value, then F is harder than O. Thus, F is like marble while O is like a football, as shown the picture. More information can be found in the Full Paper by B. C. B. Symons et al. on page 560 in Issue 5, 2019 (DOI: 10.1002/open.201800275).