Quantum effects in solid hydrogen at ultra-high pressure

We present ab initio path integral simulations of solid hydrogen in the pressure range from about 150 to 700 GPa at 50 K. Comparisons of the fully quantum-mechanical simulations with analogous ones with classical protons allow us to assess the nuclear quantum effects on the structure. Taking into account the zero-point motion in the framework of the (quasi-) harmonic approximation is shown to be insufficient at megabar pressures. The structure of solid hydrogen at very high compression is found to be unexpectedly diffuse due to quantum effects, sharing characteristics with fluid-like or even plasma-like phases. This suggests the concept of a "quantum fluxional solid", which cannot be understood in terms of a single underlying classical structure.