Understanding the behavior of matter under extreme compression remains a grand challenge in physics, planetary and materials science. It was long assumed that increasing pressure causes all matter to become metallic, comprising ionic cores arranged in close-packed lattices and free electrons . However, experimental results have shown that this view might be incomplete. Sodium, which at ambient pressure is a metal, becomes transparent to visible light when compressed above a pressure of ~120 GPa . The transparency is indicative of the opening of a band-gap, meaning the valence electrons are no longer able to move freely in the conduction bands . Indeed, the calculated band-gap for this new phase of sodium, named Na-hP4, is ~1.3 eV; therefore, sodium becomes a semiconductor under extreme compression . In fact, sodium becomes a highpressure electride (HPE), as consequence of the reordering of the valence orbitals, which form 3(s)pd hybrid orbitals, whose overlap results in the formation of a multicentered bond and concomitant localization of electron density in interstitial sites . A more simplistic model has viewed the electrons as being transferred from the Na atom to so-called interstitial quasi atoms (ISQ) . Though conceptually appealing, this model is not fully supported by detailed analysis of the electronic structure .