  The stability mechanism of cuboctahedral clusters in nonstoichiometric
uranium dioxide is investigated by first-principles LSDA+U method. Calculations
reveal that the structural stability is inherited from U6O12 molecular cluster
whereas the energy gain through occupying its center with an additional oxygen
makes the cluster win out by competition with point oxygen interstitials. Local
displacement of the center oxygen along <111> direction also leads the cluster
8-folded degeneracy and increases relatively the concentration at finite
temperatures. But totally, elevation of temperature, i.e., the effect of
entropy, favors point interstitial over cuboctahedral clusters.
