Since the dawn of quantum mechanics, it has been assumed that high-pressure matter can be described by a 1D solution for the electronic wavefunction in a spherical potential, which satisfies Fermi statistics (e.g. the TFD model -a precursor of modern DFT). This model predicts that with increasing pressure all matter becomes metallic -and is constituted by ionic cores locked on simple close-packed lattices that are bathed in a sea of free electrons, as shown schematically in Figure . The reason for this is that the kinetic energy of electrons increases more quickly than their Coulomb energy as their density is increased; yet the kinetic energy can be minimized by adopting a homogeneous density with a concomitant metallization. 8 In the traditional view the metallic system (left) will transition to a metallic sea of electrons, which can be described by TFD theory (top right). In some systems charge localization (CL -dark blue boxes) between the ionic cores (small light blue circles) is observed instead (bottom right).