The quantity r 3 a may be interpreted as the volume occupied by atom a within the molecular environment, and it is computed via Hirshfeld partition of the SCF density. The Hirshfeld technique, which is more often associated with atomic partial charges, integrates the molecular SCF density ρ(r) using a weighting function w a (r) for atom a. This function is obtained by considering where (in space) atom a contributes to the superposition of free-atom densities, b ρ free b (r), and is given by