  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 suggests that the quantum
rotor behaviour disappears with pressurisation, without any apparent phase
transition. Here we present a simple thermodynamic model for this behaviour
based on packing atoms and molecules and discuss the thermodynamics of the
phase boundaries. We also report first principles molecular dynamics
calculations for a more detailed look at the same phase transitions.
