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

\opening{Dear Editors,}

\justifying
Please find attached a revised version of the manuscript entitled 
\begin{quote}
	\textit{``Benchmarking CASPT3 Vertical Excitation Energies ''}.
\end{quote}
We thank the reviewers for their constructive comments and to support publication of the present manuscript.
Our detailed responses to their comments can be found below.
For convenience, changes are highlighted in red in the revised version of the manuscript. 

We look forward to hearing from you.

\closing{Sincerely, the authors.}

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%%% REVIEWER 1 %%%
\noindent \textbf{\large Authors' answer to Reviewer \#1}
 
{Finally, some benchmarking on the CASPT3 method. 
This should have been done long ago -- the authors are kicking in an open door. 
A few things need to be modified -- minor -- for the paper to be accepted.  
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\item 
{Table I should be moved to the SI. Key entries could be kept if explicitly discussed in the manuscript.}
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\item 
{It should be stressed that the conclusions are only valid for medium-sized organic molecules. 
The very important classes of transition metal complexes are not covered.}
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\item 
{Make it VERY clear that the excitation energies are Single-State CASPTX excitation energies.}
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\item 
{The authors should point out the difference between dynamic sigma polarization and so-called left-right polarization (https://doi.org/10.1021/jp9528020).}
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%%% REVIEWER 2 %%%
\noindent \textbf{\large Authors' answer to Reviewer \#2}
 
{This is an interesting paper that shows a benchmark of CASPT3 energies using small organic molecules. 
Such studies are important and suitable for JCP, so in principle I am in favor of it. 
I have however some comments that I would like the authors consider. 
}
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\item 
{One straightforward request is that I would have liked to see also a histogram in fig 2 of the CASSCF values of Table 1. 
CASSCF has largest errors, sure, but it would be interesting to see how broad is the distribution in comparison to caspt3, represented in the same way.}
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\item 
{My other, not so straightforward, comment refers to the IPEA correction. 
It is a valuable and welcome conclusion that the authors found that CASTP3 is less sensitive to IPEA. 
Still, in saying that CASPT3 is like CASPT2(NOIPEA) the paper seems to advocate that one should use IPEA in CASPT2. 
The first thing that authors should do is to point out that the IPEA shift is differently implemented in MOLPRO (used in this paper) than in MOLCAS (used in ref 56 which recommends not to use IPEA for the same type of organic molecules) and therefore, this apparent conclusion should be taken with caution. 
Second, this conclusion relies on the chosen theoretical best estimates (TBE). 
The TBEs (for molecules of the Thiel set, \url{https://lcpq.github.io/QUESTDB_website/subsets/}, QUEST3 subset) seems to be largely CC3, CCSDT, CCSDTQ, and NEVPT2 with aug-TZ/QZ basis sets. 
From a theoretical point of view, this should give an impressive accuracy, as they are high-order wave function based methods and large basis sets. 
And indeed, CASPT2/IPEA performs similar to CCSD/CC3/NEVPT2 excitation energies for the Thiel set. 
However, all these methods (CCSD/CC3/NEVPT2/IPEA) using the TZVP basis set overestimate experimental excitation energies, while NOIPEA underestimates excitation energies of the Thiel set, and this underestimation with the TZVP basis set was smaller in magnitude than the overestimation of the other methods, making NOIPEA with TZVP basis set preferable. 
The question is, does one want to be as good as CASPT2 or as good as the experimental value? 
I would say the latter, so while the TBEs are useful, a comparison with the experimental values should be provided and the results also discussed in this light.}
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\item 
{Can the authors also discuss if there is any basis set effect? Are the TBEs obtained with the same basis set as their calculations? 
Probably not. 
The authors could be more a bit more specific about the method/basis set of each of the TBEs in the SI, so that everyone can compare for a specific molecule (if desired) the setups in which the values where obtained. 
Again, this should be discussed against the experimental value in order to convey a clear recommendation to the readers.}
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