Small yttrium-carbon and lanthanum-carbon clusters: Rings are most stable

A theoretical study has been undertaken to determine the energetics of a variety of neutral and cationic isomeric forms of metal clusters MC(x), where M = Y or La and x = 3-6. Included in this study are cyclic molecules and linear molecules, as well as recently-proposed ''kite'' structures. Geometries are optimized by the B3LYP density functional method, and energies are computed with the coupled-cluster method. The major conclusion of this work is that cyclic structures are the most stable, a result which holds for both yttrium and lanthanum and for both cations and neutral molecules.