1. Theoretical calculations and experimental results are challenging the time-honored Thomas-Fermi-Dirac (TFD) model, which predicts that at high-pressure all matter adopts structures where the ions sit on simple closed-packed lattices, and are bathed in a metallic sea of electrons. Instead, in some cases (e.g. Cs-IV and the insulating Na-hP 4 phase) open structures become preferred under pressure, and quantum mechanical calculations suggest they are high-pressure electrides (HPEs) -systems where electrons trapped within interstitial voids act as the anions.