In the extreme case, pressure leads to metallization, i.e., equalization of the electron distribution within a substance. This behaviour was already predicted Sommerfeld and Welker, who, by studying the hydrogen atom in an impenetrable box, found that the electron becomes solely bound by the box rather than the core for small boxes and, consequently, high pressures. Below the metallization threshold, different substances respond to pressure in different ways. DFT calculations using PBCs, for example, have demonstrated that XeF 2 becomes [XeF] + F -when the pressure is high enough, i.e., the substance undergoes dissociation into an ionic solid. A curium compound, on the contrary, becomes more covalent under pressure, as evidenced by DFT and multireference calculations. How the character of a bond is influenced by pressure depends on many factors such as the crystal structure, possible interactions between metal atoms and the nature of the metal-ligand bonds.