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\begin{letter}%
{To the Editors of the Journal of Chemical Physics}

\opening{Dear Editors,}

\justifying
Please find enclosed our manuscript entitled \textit{``Benchmarking CASPT3 Vertical Excitation Energies''}, which we would like you to consider as a Regular Article in the \textit{Journal of Chemical Physics}.
This contribution has never been submitted in total nor in parts to any other journal, and has been seen and approved by all authors.

In the present article, using 284 highly-accurate reference vertical excitation energies of various natures (singlet, triplet, valence, Rydberg, $n\to\pi^*$, $\pi\to\pi^*$, and double excitations) extracted from the QUEST database, we benchmark the third-order multireference perturbation theory method, CASPT3.
Although CASPT3 calculations have been reported in the literature the present study provides, to the best of our knowledge, the first comprehensive benchmark of CASPT3 and allows assessing its accuracy in the framework of electronically excited states.
In particular, by computing vertical excitation energies with and without the infamous ionization-potential-electron-affinity (IPEA) shift and comparing these results to the second-order multireference perturbation theory method, CASPT2, we are able to show that
\begin{itemize}
\item[(i)] CASPT3 transition energies are almost independent of the IPEA shift; 
\item[(ii)] CASPT2(IPEA) and CASPT3 have a very similar accuracy. 
\end{itemize}
These global trends are also true for specific sets of excitations and various system sizes.

We suggest Stefano Battaglia, Javier Segarra-Marti, Peter Knowles, Hans-Joachim Werner, Leticia Gonz\'alez, Celestino Angeli, and Donald Truhlar as potential referees.
We look forward to hearing from you.

\closing{Sincerely, the authors.}

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