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+%% This BibTeX bibliography file was created using BibDesk.
+%% http://bibdesk.sourceforge.net/
+
+%% Created for Pierre-Francois Loos at 2019-05-13 21:13:34 +0200 
+
+
+%% Saved with string encoding Unicode (UTF-8) 
+
+
+
+@control{achemso-control,
+	Ctrl-Article-Title = {yes},
+	Ctrl-Chapter-Title = {yes},
+	Ctrl-Etal-Firstonly = {yes},
+	Ctrl-Etal-Number = {30}}

Manuscript/ExPerspective.tex

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+\documentclass[aps,prb,reprint,noshowkeys,superscriptaddress]{revtex4-1}
+\usepackage{graphicx,dcolumn,bm,xcolor,microtype,multirow,amscd,amsmath,amssymb,amsfonts,physics,mhchem,longtable,wrapfig,txfonts}
+
+\usepackage{natbib}
+\usepackage[extra]{tipa}
+\bibliographystyle{achemso}
+\AtBeginDocument{\nocite{achemso-control}}
+
+\usepackage{hyperref}
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+    colorlinks=true,
+    linkcolor=blue,
+    filecolor=blue,
+    urlcolor=blue,
+    citecolor=blue
+}
+\urlstyle{same}
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+\newcommand{\toto}[1]{\textcolor{brown}{#1}}
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+\newcommand{\trashDJ}[1]{\textcolor{purple}{\sout{#1}}}
+\newcommand{\trashAS}[1]{\textcolor{brown}{\sout{#1}}}
+
+\usepackage{hyperref}
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+    colorlinks=true,
+    linkcolor=blue,
+    filecolor=blue,
+    urlcolor=blue,
+    citecolor=blue
+}
+\newcommand{\mc}{\multicolumn}
+\newcommand{\ie}{\textit{i.e.}}
+\newcommand{\eg}{\textit{e.g.}}
+\newcommand{\ra}{\rightarrow}
+\newcommand{\fnm}{\footnotemark}
+\newcommand{\fnt}{\footnotetext}
+\newcommand{\tabc}[1]{\multicolumn{1}{c}{#1}}
+\newcommand{\SI}{\textcolor{blue}{supporting information}}
+\newcommand{\QP}{\textsc{quantum package}}
+
+% units
+\newcommand{\IneV}[1]{#1 eV}
+\newcommand{\InAU}[1]{#1 a.u.}
+\newcommand{\InAA}[1]{#1 \AA}
+\newcommand{\kcal}{kcal/mol}
+
+\newcommand{\LCPQ}{Laboratoire de Chimie et Physique Quantiques, Universit\'e de Toulouse, CNRS, UPS, France}
+\newcommand{\CEISAM}{Laboratoire CEISAM - UMR CNRS 6230, Universit\'e de Nantes, 2 Rue de la Houssini\`ere, BP 92208, 44322 Nantes Cedex 3, France}
+
+\begin{document}	
+
+\title{The Quest For Highly Accurate Excitation Energies}
+
+\author{Pierre-Fran\c{c}ois \surname{Loos}}
+\email{loos@irsamc.ups-tlse.fr}
+\affiliation{\LCPQ}
+\author{Anthony \surname{Scemama}}
+\affiliation{\LCPQ}
+\author{Denis \surname{Jacquemin}}
+\email[Corresponding author: ]{Denis.Jacquemin@univ-nantes.fr}
+\affiliation{\CEISAM}
+
+\begin{abstract}
+%\begin{wrapfigure}[13]{o}[-1.25cm]{0.5\linewidth}
+%	\centering
+%  \includegraphics[width=\linewidth]{TOC}
+%\end{wrapfigure}
+In this Perspective, we provide a global overview of the successive steps that made possible to obtain increasingly accurate excited-state energies and properties, eventually leading to chemically accurate excitation energies. 
+We describe
+i) the evolution of ab initio reference methods, e.g., originally CASPT2 (Roos, Serrano-Andres in the 1990's), then high-level CCn (as in the 
+acclaimed Thiel benchmark series in the 2000's), and now selected CI methods thanks to their resurgence in the past five years; 
+ii) how these high-level methods have allowed to assess fairly and accurately the performances of lower-order methods, e.g., ADC, 
+TD-DFT and BSE; 
+iii) the current potentiality of these various methods from both an expert and non-expert points of view; 
+iv) what we believe could be the future developments in the field.
+\end{abstract}
+
+\maketitle
+
+%%%%%%%%%%%%%%%%%%%%
+%%% INTRODUCTION %%%
+%%%%%%%%%%%%%%%%%%%%
+The accurate modelling of excited-state properties from \textit{ab initio} quantum chemistry methods is a challenging yet self-proclaimed ambition of the electronic structure theory community that will certainly keep us busy for (at the very least) the next few decades to come.
+Of particular interest is the access to precise excitation energies, \ie, the energy difference between the ground and excited states.
+The factors that make this quest for high accuracy particularly delicate are very diverse.
+First of all (and maybe surprisingly), it is, in most cases, tricky to obtain reliable and accurate experimental data that one can straightforwardly compare to theoretical values.
+In the case of vertical excitation energies, \ie, excitation energies at a fixed geometry, band maxima does not usually match theoretical values as one needs to take into account the geometric relaxation and the zero-point vibrational energy motion.
+Even more problematic, experimental spectra might not available in gas phase, and in the worst-case scenario, wrong assignments may occur.
+
+Excited states are usually extremely close in energy and can have very different nature ($\pi \ra \pi^*$, $n \ra \pi^*$, charge transfer, double excitations, Rydberg states, etc).
+Therefore one needs --- in principle at least ---  a computational method providing a balanced theoretical treatment of all these excited states.
+And let's be honest, none of the existing methods does provide this.
+
+If you were asking for the perfect excited state method for Christmas, what would you (realistically) ask for?
+The requirement for the ``perfect'' theoretical model would be:
+i) balanced treatment of excited states with different character.
+ii) chemically accurate excitation energies ie error smaller than 1 kcal/mol or $0.05$ eV.
+iii) other properties (such as oscillator strength, dipole moment,  optimization for excited tstae geometries (analytical highliy desirable)
+iv) Minimal user input requirement (black box method) and minimization of chemical intuition in order not to bias the results.
+v) Low computational scaling with respect to system size and small memory footprint.
+
+Although people usually don't really like reading, reviewing or even the idea of benchmark studies, these are definitely essential for the validation of existing theoretical methods and to understand their strengths and, more importantly, their limitations.
+
+%%%%%%%%%%%%%%%%%%%
+%%% ROOS' GROUP %%%
+%%%%%%%%%%%%%%%%%%%
+Roos' group probably kick started the whole thing thanks to their method CASPT2.
+
+%%%%%%%%%%%%%%%%%%%%%
+%%% THIEL'S GROUP %%%
+%%%%%%%%%%%%%%%%%%%%%
+A major contribution originates from the Thiel's group
+
+%%%%%%%%%%%%%%%%%%%
+%%% SCI METHODS %%%
+%%%%%%%%%%%%%%%%%%%
+In the past five years, we have witnessed a resurgence of selected CI (sCI) methods thanks to the development and implementation of new and fast algorithm to select cleverly determinants in the FCI space.
+sCI methods rely on the same principle as the usual CI approach, except that determinants are not chosen a priori based on occupation or excitation criteria but selected among the entire set of determinants based on their estimated contribution to the FCI wave function. 
+Indeed, it has been noticed long ago that, even inside a predefined subspace of determinants, only a small number of them significantly contributes.
+Therefore, an on-the-fly selection of determinants is a rather natural idea that has been proposed in the late 1960s by Bender and Davidson19 as well as Whitten and Hackmeyer.s
+sCI methods are still very much under active development. 
+The main advantage of sCI methods is that no a priori assumption is made on the type of electronic correlation. 
+Therefore, at the price of a brute force calculation, a sCI calculation is less biased by the user's appreciation of the problem's complexity.
+The approach that we have implemented in QUANTUM PACKAGE is based on the CIPSI algorithm developed by Huron, Rancurel, and Malrieu in 1973.
+
+
+In Ref., we studied 18 small molecules (water, hydrogen sulfide, ammonia, hydrogen chloride, dinitrogen, carbon monoxide, acetylene, ethylene, formaldehyde, methanimine, thioformaldehyde, acetaldehyde, cyclopropene, diazomethane, formamide, ketene, nitrosomethane, and the smallest strepto- cyanine) with sizes ranging from one to three nonhydrogen atoms. 
+For such systems, using sCI expansions of several million determinants, we were able to compute more than 100 highly accurate vertical excitation energies with typically augmented triple-$\zeta$ basis sets. 
+It allowed us to benchmark a series of 12 state-of-the-art excited-state wave function methods accounting for double and triple excitations.
+Even more recently, we provided accurate reference excitation energies for transitions involving a substantial amount of double excitation using a series of increasingly large diffuse-containing atomic basis sets. 
+Our set gathered 20 vertical transitions from 14 small- and medium-sized molecules (acrolein, benzene, beryllium atom, butadiene, carbon dimer and trimer, ethylene, formaldehyde, glyoxal, hexatriene, nitrosomethane, nitroxyl, pyrazine, and tetrazine). 
+For the smallest molecules, we were able to obtain well converged excitation energies with an augmented quadruple-$\zeta$ basis set, while only augmented double-$\zeta$ bases were manageable for the largest systems (such as acrolein, butadiene, hexatriene, and benzene). 
+Note that the largest sCI expansion considered in this study had more than 200 million determinants.
+
+%%%%%%%%%%%%%%%%%%
+%%% CONCLUSION %%%
+%%%%%%%%%%%%%%%%%%
+
+%%%%%%%%%%%%%%%%%%%%%%%%
+%%% ACKNOWLEDGEMENTS %%%
+%%%%%%%%%%%%%%%%%%%%%%%%
+PFL acknowledges funding from the \textit{``Centre National de la Recherche Scientifique''}.
+DJ acknowledges the R\'egion des Pays de la Loire for financial support. 
+
+%%%%%%%%%%%%%%%%%%%%
+%%% BIBLIOGRAPHY %%%
+%%%%%%%%%%%%%%%%%%%%
+\bibliography{ExPerspective,ExPerspective-control}
+
+\end{document}