Yet, these orbitals are clearly important at the pressures where Na-hP 4 is stable, as seen in the projected DOS plot of Figure . In fact, in their seminal paper Neaton and Ashcroft discussed how the combined effects of Pauli exclusion and orthogonality would drive an electronic s → pd transfer in compressed sodium, predicting its eventual metal-to-insulator transition, and later DFT calculations have shown that the p and d states become important, for example by plotting the orbital projected DOS. However, none of these studies provided a visual orbital based picture that explains how the s → pd electronic reorganization leads to interstitial electron localization within Na-hP 4. Such a picture was obtained when the NNM-centered Wannier-like function that described the valence band of Na-hP 4 was computed. This Wannier-like function, illustrated in Within this second model of the electronic structure of HPEs, the electron density situated in the voids is caused by the constructive overlap of atom-centered orbitals. These orbitals are not occupied at 1 atm, but their energy becomes lower than that of the Na 3s states under pressure, since they are orthogonal to the core 1s and 2s, and therefore experience lower Pauli repulsion in the compressed solid. Basis functions with higher angular momenta seem to be crucial for the formation of the HPE phase of sodium (but this is not a general rule, as we have seen in the case of Tl), by providing the extra "directionality" away from the nearest neighbor contact between two Na atoms, which would be too close to form stable chemical bonds. Therefore, just like in metal ammonia solutions, the electride state in compressed matter can also be understood in terms of MCB formation. Previous work has shown that the overlap of sd hybrid orbitals, also obtained by constructing Wannier-like functions, can explain the formation of the electride state in Cs-IV. Though the electronic structure of other elemental HPEs have been discussed in this context, 115 Wannier-like functions that describe their occupied band have not been computed, and it would be desirable to extend such analyses to a broader range of systems. Nonetheless, just like in the various species hypothesized to comprise metal ammonia solutions, the charge build-up in the interstices of HPEs can be attributed to MCB formation.