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Quantum Monte Carlo Study of High Pressure Solid Molecular Hydrogen
We use the diffusion quantum Monte Carlo (DMC) method to calculate the ground
state phase diagram of solid molecular hydrogen and examine the stability of
the most important insulating phases relative to metallic crystalline molecular
hydrogen. We develop a new method to account for finite-size errors by
combining the use of twist-averaged boundary conditions with corrections
obtained using the Kwee-Zhang-Krakauer (KZK) functional in density functional
theory. To study band-gap closure and find the metallization pressure, we
perform accurate quasi-particle many-body calculations using the method.
In the static approximation, our DMC simulations indicate a transition from the
insulating Cmca-12 structure to the metallic Cmca structure at around 375 GPa.
The band gap of Cmca-12 closes at roughly the same pressure. In the
dynamic DMC phase diagram, which includes the effects of zero-point energy, the
Cmca-12 structure remains stable up to 430 GPa, well above the pressure at
which the band gap closes. Our results predict that the semimetallic state
observed experimentally at around 360 GPa [Phys. Rev. Lett. {\bf 108}, 146402
(2012)] may correspond to the Cmca-12 structure near the pressure at which the
band gap closes. The dynamic DMC phase diagram indicates that the hexagonal
close packed structure, which has the largest band gap of the
insulating structures considered, is stable up to 220 GPa. This is consistent
with recent X-ray data taken at pressures up to 183 GPa [Phys. Rev. B {\bf 82},
060101(R) (2010)], which also reported a hexagonal close packed arrangement of
hydrogen molecules
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