Non-bonded atomic radii, for example, have been studied with XP-PCM up to 300 GPa. The relative ordering of atomic radii present at ambient conditions was found to be largely retained at higher pressure, with an increase in pressure in most cases leading to a smooth decrease in the atomic radius. A notable exception to this rule is lithium: At 209 GPa, a marked decrease in the atomic radius was observed, which was explained by a change in the electronic configuration from 1s 2 2s 1 to 1s 2 2p 1 . This demonstrates that electronic configuration and atomic radius are intimately connected, an effect that is demonstrated clearly also in the iron atom (Figure ).