To obtain the most stable C-2-C52X2 (X = H, F, and Cl) isomers, all 676 possible C-2-C52H2 isomers are orderly optimized using PM3, HCTH/3-21G, and B3LYP/6-31G* methods. The C-2-C52X2 (X = F and Cl) being isostructural with the first 19 most stable C-2-C52H2 are regarded as candidates of the most stable isomers, and optimized using B3LYP/6-31G*, 28,52- and 26,52-C-2-C52X2 (X = H, F, and Cl) are the two most stable isomers, and stability-order of isomers is influenced by the properties of addends. T-C52X2 (X = H, F, and Cl) are also optimized for comparison. (C) 2008 Elsevier B.V. All rights reserved.