Theoretical studies that employ ab initio density functional techniques have been presented for the relative stabilities of isomers of LaC + 1 2 and LaC + 1 3 . While LaC + 1 2 exhibits the most stable isomer with La inserted into the carbon ring, LaC + 1 3 exhibits the most stable structure in which the La atom is inside the ring. Our computations are consistent with the recent experimental study on LaC + n isomers which suggests alternation of fast and slow ring isomers as a function ofn .