document: The early theoretical work of Catlow assessed a number of Willis type clusters and found them all to be stable using potential-based methods [6]. More recently “split interstitial” type clusters (Fig. 1) have emerged from computational studies as stable species following the potential based investigation of Govers et al. which found the 2:2:2 cluster in a UO2 supercell relaxed to a split di-interstitial [13] (Fig. 1(b)); a single VO with three Oi displaced approximately 1.6Å in 〈111〉 directions from the VO. This result was later confirmed by the LSDA+U calculations of Geng et al. [7]. The family of split interstitial clusters was extended to include tri-interstitials [8] (a di-interstitial with the fourth Oi site occupied) and quad-interstitials [9] (two di-interstitials on adjacent sites, giving a total of two VO and six Oi) (Fig. 1(d)). Following this Andersson et al. postulated a model for U4O9 based on a UO2 supercell containing multiple split quad-interstitial clusters; following the prediction of their LSDA+U calculations that the quad-interstitial is more stable than its cuboctahedral counterpart [12].
extractive_keyphrases: a single vo with three oi cuboctahedral model for u4o9 oi potential based investigation split interstitial clusters uo2 supercell vo willis type clusters
abstractive_keyphrases: di-interstitial di-interstitial with the fourth oi site occupied lsda+u multiple split quad-interstitial clusters potential-based methods quad-interstitial quad-interstitials split interstitial” type clusters tri-interstitials two di-interstitials on adjacent sites