A three degree-of-freedom potential energy surface of the cyclooctatetraene (COT) unimolecular reaction that can describe both ring-inversion (D-2d <-> D-2d) and double bond-alternation (D-4h <-> D-4h) processes was constructed using complete active space self-consistent field calculations. The potential energy surface was used to simulate the experimentally measured transition-state spectrum by calculating the photodetachment spectrum of the COT anion with time-dependent wave packet formalism. The calculated spectrum reproduces the experimental result well. We also analyzed wavefunction properties at spectral peak positions to understand the COT unimolecular reaction dynamics. (C) 2015 Elsevier B.V. All rights reserved.