We look first at the sizes of atoms (and ions) inside our collection of materials. Defining atomic boundaries in open (e.g., molecular) systems is not trivial, but can, for instance, be done successfully using different cut-off criteria: The electron density and properties of valence orbitals are some of the possibilities. For crystals, there exist other options. We here choose to attribute atomic volumes 𝑉 7 directly to the volume of electron energy basins 𝛺 7 .