Fig. 3
CCSD relative enthalpies. The CCSD relative enthalpies of structures that are used in this paper (dashed lines) and the DMC relative enthalpies of structures from ref. 2 (full lines). The thickness of the full lines refer to the standard deviations of stochastic sampling of the 1st Brillouin zone while performing twist-averaging in the DMC calculations. In this work, the 1st Brillouin zone is sampled using a dense regular grid such that the errors are converged to within 1 meV/atom. See supplementary information for details. CCSD and DMC2 agree very well in the most stable molecular phases, i.e., C2/c-24, P2\({}_{1}\)/c-24 and Cmca-12, whilst the only discrepancy exists in the Cmca-4 phase, which is predicted by DFT-PBE to be metallic at high pressures. The phase transition between P6\({}_{3}\)/m-16 and C2/c-24 predicted by CCSD happens at ~350 GPa, which agrees reasonably well with the DMC transition pressure range 250–350 GPa from ref. 3
