Low-Lying Isomers of (TiO_2)_n (n=2-8) Clusters

Although there are diverse bond features of Ti and O atoms, so far only several isomers have been reported for each (TiO_2)_n cluster. Instead of the widely used global optimization, in this work, we search for the low-lying isomers of (TiO_2)_n (n=2-8) clusters with up to 10000 random sampling initial structures. These structures were optimized by the PM6 method, followed by density functional theory calculations. With this strategy, we have located many more low-lying isomers than those reported previously. The number of isomers increases dramatically with the size of the cluster, and about 50 isomers were found for (TiO_2)_7 and (TiO_2)_8 with the energy within kcal/mol. Furthermore, new lowest isomers have been located for (TiO_2)_5 and (TiO_2)_8, and isomers with three terminal oxygen atoms, five coordinated oxygen atoms as well as six coordinated titanium atoms have been located. Our work highlights the diverse structural features and a large number of isomers of small TiO_2 clusters.