submitted to JPCA

Manuscript/CBD.tex

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 \documentclass[aip,jcp,reprint,noshowkeys,superscriptaddress]{revtex4-1}
 \usepackage{graphicx,dcolumn,bm,xcolor,microtype,multirow,amscd,amsmath,amssymb,amsfonts,physics,longtable,wrapfig,txfonts}
 \usepackage[version=4]{mhchem}
-%\usepackage{natbib}
+\usepackage{natbib}
-%\bibliographystyle{achemso}
+\bibliographystyle{achemso}
 \usepackage[utf8]{inputenc}
 \usepackage[T1]{fontenc}
 \usepackage{txfonts}

Manuscript/Cover_Letter/CoverLetter.tex

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 \documentclass[10pt]{letter}
-\usepackage{UPS_letterhead,xcolor,mhchem,mathpazo,ragged2e}
+\usepackage{UPS_letterhead,xcolor,mhchem,ragged2e}
 \newcommand{\alert}[1]{\textcolor{red}{#1}}
 \definecolor{darkgreen}{HTML}{009900}
 
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 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.
+
 We suggest David Casanova, Miquel Huix-Rotllant, Michael Filatov, Donald Truhlar, and Andreas Dreuw as potential referees.
 We look forward to hearing from you.