Electronic Structure Determines Geometry: Bond Length Alternating in Cyclo[2n]carbons

Structure determines the properties. However, whether electronic structure determines geometry or geometry determines electronic structure seems a philosophical question in a chicken and egg situation, which remains unclear. In this work, by applying density functional theory (DFT) and DMRG(4n,4n)-CASSCF methods, theoretical investigation suggested that the dual antiaromaticity in cyclo[2n]carbons with even nshould be attributed to the electron correlation effect, instead of decreased geometric symmetry, which actually exists in all cyclo[2n]carbon molecules and does not point out the essence. Such dual antiaromaticity can be conceptualized as electron correlation-stabilized dual antiaromaticity. Results also showed that DFT is reliable for cyclocarbons larger than C14, but we should be careful when applying it to smaller ones. DFT failed to give the correct structure of C6compared with density matrix renormalization group results.