The Equilibrium CC Bond Length

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