“…Numerous electronic structure calculations of cyclo [18]carbon molecule have been carried out at the different levels of theory including the HF, 18,20 SCF, [21][22][23] MCSCF, 24 MP2, 21,22 DFT, 19,21,[25][26][27][28][29][30][31][32] quantum Monte Carlo (QMC) 24 and CCSD 23,29 levels. Surprisingly, only the HF, QMC and CCSD methods yield the polyyne structure that experimentally has been observed in high-resolution atomic force microscopy (AFM) measurements of cyclo [18]carbon adsorbed on bilayer NaCl on Cu(111) surface at the temperature of 5 K. 16 Most of the common DFT functionals 19,21,[25][26][27][28][29][30][31][32] (B3LYP, BLYP, PBE0, etc.) as well as MP2 calculations 21 yield the nonbond-length alternating cumulene structure of cyclo [18]carbon.…”