Theory of high pressure hydrogen, made simple

Phase I of hydrogen has several peculiarities. Despite having a close-packed crystal structure, it is less dense than either the low temperature Phase II or the liquid phase. At high pressure, it transforms into either phase III or IV, depending on the temperature. Moreover, spectroscopy suggest that the quantum rotor behaviour disappears with pressurisation, without any apparent phase transition. Here we report molecular dynamics (MD) calculations which show that all these phenomena can be understood in a classical model with simple treatment of exchange-correlation. We also discuss the thermodynamics of the phase boundaries of Phase I.