{To the Editors of the Journal of Physical Chemistry A,}
\opening{Dear Editors,}
\justifying
Please find enclosed our manuscript entitled \textit{``Reference Energies for Cyclobutadiene: Automerization and Excited States''}, which we would like you to consider as a Regular Article in the \textit{Journal of Physical Chemistry A}.
This contribution has never been submitted in total nor in parts to any other journal, and has been seen and approved by all authors.
The cyclobutadiene molecule has been a real challenge for both experimental and theoretical chemistry for many decades.
In particular, cyclobutadiene is a well-known playground for theoretical chemists and is particularly suitable to test ground- and excited-state methods.
In the $D_{2h}$ symmetry, the ground state has a weak multi-configurational character with well-separated frontier orbitals that can be described by single-reference methods.
However, in the $D_{4h}$ symmetry, the ground state is a diradical that has two degenerate singly occupied frontier orbitals.
In the present paper, we assess a larger number of computational methods on the automerization barrier and the vertical excitation energies of cyclobutadiene in its square ($D_{4h}$) and rectangular ($D_{2h}$) geometries, for which we have defined theoretical best estimates based on high-level coupled-cluster calculations.
In particular, selected configuration interaction, multi-reference perturbation theory (CASSCF, CASPT2, and NEVPT2), and coupled-cluster calculations are performed.
These wave function methods are compared with the cheaper spin-flip formalism which is tested within TD-DFT and the ADC scheme.
Several important take-home messages are extracted from the present study, such as the excellent performance of hybrid functionals with a large amount of exact exchange, the outstanding accuracy of the ADC schemes, the importance of the $\sigma$-$\pi$ coupling in multi-configurational calculations, and the importance of quadruple excitations to reach high accuracy in coupled-cluster theory.
