The structures of three possible C-60 dimers with intact cages have been calculated at the semiempirical (MNDO/ AM-1, PM-3) and ab initio density functional theory (DFT) levels. At the DFT level, both local (LDFT) and nonlocal (NLDFT) calculations have been performed. The dimers are formed by the 1,2-, 1,4-, and 1,2+1,4-cycloadditions of a cyclohexatrienyl unit of each C-60. The energies of the dimers relative to that of the most stable 1,2-C-60 dimer are 74.0 kcal/mol (AM-1), 75.2 kcal/mol (PM-3), 69.7 kcal/mol (LDFT), and 76.2 kcal/mol (NLDFT) for the 1,4-C-60 dimer and 93.0 kcal/mol (AM-1), 87.0 kcal/mol (PM-3), 69.8 kcal/mol (LDFT), and 69.5 kcal/mol (NLDFT) for the 1,2+1,4-C-60 dimer. The dimerization reaction to form the C-60 dimers from C-60 is calculated to be weakly endothermic only for the formation of 1,2-(C-60)(2), and for the others, the reaction is predicted to be strongly endothermic. This result suggests that 1,2-cycloaddition is the most plausible scheme for the dimerization and polymerization of C-60 molecules, if the cages remain intact.