From the MAO perspective, there are two σ bonds involving Be, one with H and one with Cl. Each is made from sp hybrid orbitals on Be, meaning that the p orbitals of Be are at play in this σ bonding, as shown in Fig. . These bonds are both polarized away from Be, as expected. The origin of the much larger IAO charge can now be understood. The IAO reference minimal basis set, known as 'MINAO', 109 does not include 2p orbitals for Be, and therefore we are instead seeing essentially only the Be(2s) charge via the IAO approach! Iterative Hirshfeld evidently struggles with a similar issue, leading to similar overestimation of Be charge. Overall, this case nicely illustrates the advantages of the MAO population scheme that is based entirely on the system at hand, rather than some reference atomic orbitals or states. Next we study the hypervalent molecule, SF 6 , which has O h symmetry, and whose chemical bonding has long been of interest. While empty 3d functions on sulfur are needed to form 6 equivalent sp 3 d 2 hybrids, the energetic cost of promoting electrons to the 3d shell is too high for d-orbital participation in the bonding to be chemically important. Rather, the bonding may be thought of as resonance between Lewis structures with 4 covalent S-F bonds, and 2 F -anions, with a formal charge of +2 on S. 118 Using Boys localization produces 6 equivalent σ SF orbitals, as shown at the left of Fig. . As expected, these σ SF bonds are strongly polarized towards the more electronegative fluorine atom. The AB2 antibonding orbitals, also shown on the left of Fig. are fascinating because contrary to simple chemical expectations, they are not strongly polarized towards the sulfur atom.