With this in mind, let our discussion return to Na-hP 4, arguably the most iconic HPE. This famous phase was first predicted theoretically, then accessed experimentally. Perhaps the most remarkable aspect of this discovery was the transformation of a 1 atm metal via pressure to a transparent insulator. Though other metastable electride phases of solid and liquid sodium have been theoretically proposed, 101,102 evidence for Na-hP 4 has been obtained in ramp-compression experiments up to 480 GPa 103 at thousands of degrees Kelvin, and at low temperatures it is predicted to be the most stable phase until 1.75 TPa. 104 At 15.5 TPa, the global minimum predicted at zero temperature was a bcc structure with Na 12 icosahedra, cI24, which was classified as being a metallic electride, based on its band structure and charge density difference diagrams. An analysis similar to that performed above for Ca (Figure ) confirmed that Na-hP 4 fulfilled all of the topological requirements listed in Sec. , with a total of ∼1.1 electrons integrated in the NNM basin at 200 GPa (HSE06 value), in addition to a non-negligible band-gap (1.3 eV) at the same pressure. Thus, Na-hP 4 is unambiguously an electride.