Abstract: C₂ and LaC₂⁺ were studied using Hartree Fock(HF), B3LYP (Becke 3-paremeter-Lee-Yang-Parr) density functional method, second order M φller-Plesset perturbation(MP2) and coupled cluster singles and doubles with non-iterative triples(CCSD(T)) methods. The basis set employed was LANL1DZ. Geometries, vibrational frequencies and other quantities were reported. The results showed that for C₂, all the methods performed well for low spin state(singlet), while only HF and B3LYP remained so for high spin state(triplet). For LaC₂⁺, four isomers were presented and fully optimized. The results suggested that linear isomers with C_∞v and D_∞h symmetries were predicted to be saddle points on the energy surface for all the methods, while for isomers with C_2v and C_s symmetries, they were local minima except C_2v at B3LYP level, and were isoenergetic at HF, MP2 and CCSD(T) levels, near isoenergetic at B3LYP level. From the differences between HOMO and LUMO, it is also known that the isomers with C_2v and C_s symmetries offer the largest values and therefore correspond to the most stable structure. ForLa—C bond lengths, B3LYP gives the shortest, the order is B3LYP<HF<MP2<CCSD(T) for all the methods.

Key words: C₂, LaC₂⁺, Structure, Vibrational frequency, Computational methods