The equilibrium CC bond length

Twenty eight CC bonds lengths (from a sample of 23 molecules! whose experim ental equilibrium structures are thought to be reliable are calculated ab i nitio. Two different correlated methods are compared: MP2 and CCSD(T). The convergence of Dunning's correlation consistent polarized valence basis set s, cc-pVnZ is also studied. With the CCSD(T)/cc-pVQZ method, a systematic c orrection of about 0.0034 Angstrom is necessary when the frozen core approx imation is used. With this offset of 0.0034 Angstrom taken into account, th e accuracy (standard deviation) is 0.0013 Angstrom. With the cc-pVTZ basis set, the correction increases to 0.0066 Angstrom and the accuracy drops to 0.0018 Angstrom. The MP2 method is significantly less accurate and the offs et only remains constant for similar bonds. During this work, new structure s are calculated for methyl cyanide, oxirane, cyclopropenylidene, allene, a nd cyclopropene.