The instability of the cylindrical D-6h structure in [6]cycloparaphenylene, the non-zero dihedral angles between adjacent paraphenylene units, is investigated from the viewpoint of the pseudo Jahn-Teller effect, wherein the driving forces are orbital vibronic couplings of the deformation modes between occupied and unoccupied orbitals. We show that the overlap densities between the frontier orbitals including the highest occupied molecular orbital (HOMO) and a certain unoccupied/occupied molecular orbital mainly contribute to the instability. The results of vibronic coupling density analysis justify the strong coupling of the unoccupied orbital with the HOMO. (C) 2014 Elsevier B.V. All rights reserved.