Modified coupled pair functional structure

Two recent ab-initio studies are devoted to the determination of the ground-state structure of Cg. Parasuk and AlmlOf concluded that the cyclic polyacetylenic structure ( Ag) and the linear cumulenic g state are essentially isoenergetic [260]. However, the choice of basis sets and methods (MRCI modified coupled-pair functional, MCPF) heavily affects the difference in energy between the linear and cyclic forms of Cg, and so the results are quite uncertain. On the other hand, Slanina et al. proposed a nonplanar Z)2d symmetrical cyclic structure as the minimum-energy structure of Cg [261]. According to their MP2/6-31G calculations, this form is 13 kcal/mol lower in energy than the planar Qa structure. Unexpectedly, all the C-C bond lengths in the species are equivalent (1.339 A). DFT calculations favor the linear structure as compared with a planar cyclic one, but nonplanar species seem to have not been considered in this study [236]. 

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