In atomic charge in the next section). Atoms that we anticipate to be positive in a compound are smaller, while those that are negative are larger. Ionically or covalently bonded atoms also shrink noticeably compared to their non-bonded (𝑉 ?@A ) references. Potassium stands out, as it is the only atom that expands relative to its vdW-like gas-phase reference. However, our comparisons do not consider the effect of packing of spherical atoms. In a body-centered-cubic phase such as K, the packing efficiency is ∼68%. In other words, the per atom volume in a packed vdW-reference for K is 79 Å 3 , which is larger than the atomic volume of the real solid.