  The (111), (110), and (001) surfaces properties of PuO2 are studied by using
density-functional theory+U method. The total-energy static calculations
determine the relative order of stability for low-index PuO2 surfaces, namely,
O-terminated (111) > (110) > defective (001) > polar (001). The effect of
thickness is shown to modestly modulate the surface stability and chemical
activity of the (110) surface. The high work function of 6.19 eV indicates the
chemical inertia of the most stable (111) surface, and the surface O-vacancy
with concentration C_V=25% can efficiently lower the work function to 4.35 eV,
which is a crucial indicator of the difference in the surface chemical
activities between PuO2 and \alpha-Pu2O3. For the polar (001) surface, 50%
on-surface O-vacancy can effectively quench the dipole moment and stabilize the
surface structure, where the residual surface oxygen atoms are arranged in a
zigzag manner along the <100> direction. We also investigate the relative
stability of PuO2 surfaces in an oxygen environment. Under oxygen-rich
conditions, the stoichiometric O-terminated (111) is found to be the most
stable surface. Whereas under O-reducing conditions, the on-surface O-vacancy
of C_V = 1/9 is stable, and for high reducing conditions, the (111) surface
with nearly one monolayer subsurface oxygen removed (C_V = 8/9) becomes most
stable.
