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Reference Energies for Cyclobutadiene: Automerization and Excited States

Abstract : Cyclobutadiene is a well-known playground for theoretical chemists and is particularly suitable to test ground-and excited-state methods. Indeed, due to its high spatial symmetry, especially at the D 4h square geometry but also in the D 2h rectangular arrangement, the ground and excited states of cyclobutadiene exhibit multi-configurational characters and single-reference methods, such as adiabatic time-dependent density-functional theory (TD-DFT) or equation-of-motion coupled cluster (EOM-CC), are notoriously known to struggle in such situations. In this work, using a large panel of methods and basis sets, we provide an extensive computational study of the automerization barrier (defined as the difference between the square and rectangular ground-state energies) and the vertical excitation energies at D 2h and D 4h equilibrium structures. In particular, selected configuration interaction (SCI), multi-reference perturbation theory (CASSCF, CASPT2, and NEVPT2), and coupled-cluster (CCSD, CC3, CCSDT, CC4, and CCSDTQ) calculations are performed. The spin-flip formalism, which is known to provide a qualitatively correct description of these diradical states, is also tested within TD-DFT (combined with numerous exchange-correlation functionals) and the algebraic diagrammatic construction [ADC(2)-s, ADC(2)-x, and ADC(3)] schemes. A theoretical best estimate is defined for the automerization barrier and for each vertical transition energy.
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https://hal.archives-ouvertes.fr/hal-03638102
Contributor : Pierre-Francois Loos Connect in order to contact the contributor
Submitted on : Tuesday, April 12, 2022 - 9:18:44 AM
Last modification on : Thursday, July 28, 2022 - 9:37:31 PM
Long-term archiving on: : Wednesday, July 13, 2022 - 6:27:17 PM

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Enzo Monino, Martial Boggio-Pasqua, Anthony Scemama, Denis Jacquemin, Pierre-Francois Loos. Reference Energies for Cyclobutadiene: Automerization and Excited States. Journal of Physical Chemistry A, American Chemical Society, 2022, 126 (28), pp.4664-4679. ⟨10.1021/acs.jpca.2c02480⟩. ⟨hal-03638102⟩

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