  The ground state electronic structure and magnetic behaviors of curium
dioxide (CmO$_{2}$) are controversial. In general, the formal valence of Cm
ions in CmO$_{2}$ should be tetravalent. It implies a $5f^{6.0}$ electronic
configuration and a non-magnetic ground state. However, it is in sharp contrast
with the large magnetic moment measured by painstaking experiments. In order to
clarify this contradiction, we tried to study the ground state electronic
structure of CmO$_{2}$ by means of a combination of density functional theory
and dynamical mean-field theory. We find that CmO$_{2}$ is a wide-gap charge
transfer insulator with strong 5$f$ valence state fluctuation. It belongs to a
mixed-valence compound indeed. The predominant electronic configurations for Cm
ions are $5f^{6.0}$ and $5f^{7.0}$. The resulting magnetic moment agrees quite
well with the experimental value. Therefore, the magnetic puzzle in CmO$_{2}$
can be appropriately explained by the mixed-valence scenario.
