QUANTUM EFFECTS IN SOLID HYDROGEN AT ULTRA-HIGH PRESSURE

We present ab initio path integral simulations of solid hydrogen in th e pressure range from about 150 to 700 GPa at 50 K. Comparisons of the fully quantum-mechanical simulations with analogous ones with classic al protons allow us to assess the nuclear quantum effects on the struc ture. Taking into account the zero-point motion in the framework of th e (quasi-) harmonic approximation is shown to be insufficient at megab ar pressures. The structure of solid hydrogen at very high compression is found to be unexpectedly diffuse due to quantum effects, sharing c haracteristics with fluid-like or even plasma-like phases. This sugges ts the concept of a ''quantum fluxional solid'', which cannot be under stood in terms of a single underlying classical structure. (C) 1998 Pu blished by Elsevier Science Ltd. All rights reserved.