  The metal-insulator and spin state transitions of CoO under high pressure are
studied by using density functional theory combined with dynamical mean-field
theory. Our calculations predict that the metal-insulator transition in CoO is
a typical orbital selective Mott transition, where the $t_{2g}$ orbitals of Co
3d shell become metallic firstly around 60 GPa while the $e_g$ orbitals still
remain insulating until 170 GPa. Further studies of the spin states of Co 3d
shell reveal that the orbital selective Mott phase in the intermediate pressure
regime is mainly stabilized by the high-spin state of the Co 3d shell and the
transition from this phase to the full metallic state is driven by the
high-spin to low-spin transition of the Co$^{2+}$ ions. Our results are in good
agreement with the most recent transport and x-ray emission experiments under
high pressure.
