  The aim of this study is to clarify the physics which governs the transition
from epsilon phase to zeta phase of solid oxygen observed experimentally at 96
GPa using density functional theory (DFT). The transition was predicted at 40
GPa with PBE functional. Then the Hubbard correction was added to enhance the
localization of p-orbital of oxygen. The epsilon-zeta transition pressure was
significantly improved to 70 GPa. Finally, we included the non-local van der
Waals correction. The transition pressure slightly increases to 80 GPa. These
results demonstrate that the contribution from Hubbard term is superior to van
der Waals term.
