The most influential model to date was put forward by Miao and Hoffmann, 108-110 who thought of the voids in HPEs as being occupied by interstitial-quasi-atoms (ISQs) -quasispherical regions of space that lack a nucleus, but where atomic orbitals can be placed (Figure ). In this way, the ISQ model of HPEs is reminiscent of Jortner's model of metal ammonia solutions, though the forces that drive the electrons to fill the voids is different in these two classes of electrides. Miao and Hoffmann argued that HPEs can form when the energies of the atom-centered valence orbitals become higher than that of the 1s orbital of the ISQ, resulting in the transfer of charge from the atoms to the interstices. As discussed thoroughly in Ref., 108 various models show that the orbital energies of atoms increase with pressure, but their rate of increase is not the same, with s increasing more quickly than p, which in-turn increase more quickly than d. The energy of the 1s orbital on the ISQ increases as well, but its rate of increase is smaller than that of the atom-centered s orbitals, making electron transfer to it an appealing option, provided that no other atom-centered orbitals possess a lower energy at that pressure. However, Miao and Hoffmann cautioned that because of the slow increase of the energy of d orbitals, elements such as the heavier alkali metals and alkaline earths will not form ISQs because of the pressure induced s → d transition, as described for Cs in Sec. . Calculations using a helium matrix to exert pressure on isolated atoms led to the conclusion that all of the light alkali metals, Al, Mg, Si, Tl, In and Pb would possess ISQs at pressures below 500 GPa, therefore behaving as HPEs. In the heavy alkali metals, on the other hand, s → d backdonation would be energetically preferred over charge transfer to the ISQ. However, it should be emphasized that a system can be a HPE without the presence of an ISQ, a case-in-point being Cs-IV (Figure ). Moreover, the pressure induced transfer of electrons to higher-lying orbitals does not not necessarily mean an electride will form. For example, the phase of Tl experimentally shown to be stable at 4.3 TPa was calculated to be undergo an sp → d electronic transition, behaving like a pd metal without electride characteristics. 111 Therefore, neither the presence of ISQs nor the pressure induced transfer of electrons to orbitals that are not normally occupied at 1 atm can foreshadow which systems will become HPEs. To round out this manuscript, we return to where we began: the Na-hP 4 phase that has been accessed both in static and dynamic compression experiments, and whose electronic structure fulfills the electride-state check-list. Application of ISQ theory suggested that sodium would readily become an electride under pressure because the ISQ s orbital was computed to have a lower energy than the Na 3s, already at a pressure as low as 100 GPa. Miao and Hoffmann, however, did not calculate the pressure-dependent energies of p and d orbitals, which were assumed to be too high in energy to become occupied under pressure.