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\author{Xavier Blase}
	\email{xavier.blase@neel.cnrs.fr}
	\affiliation[NEEL, Grenoble]{Universit\'e Grenoble Alpes, CNRS, Institut NEEL, F-38042 Grenoble, France}
\author{Ivan Duchemin}
	\affiliation[CEA, Grenoble]{Universit\'e Grenoble Alpes, CEA, IRIG-MEM-L Sim, 38054 Grenoble, France}
\author{Denis Jacquemin}
	\affiliation[CEISAM, Nantes]{Universit\'e de Nantes, CNRS,  CEISAM UMR 6230, F-44000 Nantes, France}
\author{Pierre-Fran\c{c}ois Loos}
	\email{loos@irsamc.ups-tlse.fr}
	\affiliation[LCPQ, Toulouse]{Laboratoire de Chimie et Physique Quantiques, Universit\'e de Toulouse, CNRS, UPS, France}

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	\title{A Chemist Guide to the Bethe--Salpeter Formalism}
\date{\today}

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%%%%%%%%%%%%%%%%
%%% ABSTRACT %%%
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\begin{abstract}
Here goes the abstract
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\noindent

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%%% INTRODUCTION %%%
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In its press release announcing the attribution of the 2013 Nobel prize in Chemistry to Martin Karplus, Michael Levitt and Arieh Warshel, the Royal Swedish Academy of Sciences concluded that ``Today the computer is just as important a tool for chemists as the test tube. Simulations are so realistic that they predict the outcome of traditional experiments." Martin Karplus Nobel lecture moderated this bold statement, introducing his presentation by a 1929 quote from Dirac emphasizing that laws of quantum mechanics are "much too complicated to be soluble", urging the scientist to develop "approximate practical methods." This is where the methodology community stands, attempting to develop robust approximations to study with increasing accuracy the properties of complex systems. The study of charged or neutral electronic excitations in condensed matter systems, from molecules to extended solids, has witnessed the development of a large number of such `àpproximate" methods


%% GW historical

\cite{Hedin_1965}

%% BSE historical

\cite{Salpeter_1951}
The use of the BSE formalism in condensed-matter physics emerged in the early 50s at the semi-empirical tight-binding level with the study of the optical properties of excitonic \cite{Sham_1966,Strinati_1984,Delerue_2000}
A decade latter, the first \textit{ab initio} implementations , starting with small clusters \cite{Onida_1995,Rohlfing_1998} before addressing the case of extended solids such as semmiconductors and wide-gap ionic insulators (Li$_2$O, LiF, MgO), \cite{Albrecht_1997,Benedict_1998} and simple surfaces, \cite{Rohlfing_1999}
paved the way to the popularization in the solid-state physics community of the Bethe-Salpeter equation formalism. 


chemistry oriented reviews with, e.g.,  the language of localized basis and resolution-of-the-identity techniques, \cite{Ren_2012} or applications related to organic molecular systems, photoelectrochemistry, etc. \cite{Ping_2013,Leng_2016,Blase_2018}

large molecular benchmarks with comparisons to TD-DFT and higher level wavefunctions techniques such as CC3 \cite{Korbel_2014,Jacquemin_2015a,Bruneval_2015,Jacquemin_2015b,Hirose_2015,Jacquemin_2017,Krause_2017,Gui_2018}


charge transfer 
classification into local-, Rydberg-, or charge transfer-type 
\cite{Hirose_2017} ad developed extensively in to the TD-DFT community.


\noindent {\textbf{The computational challenge.}} \\

pooling empty state with common energy denominator techniques, \cite{Bruneval_2008}
replacing the sum over unoccupied states by iterative techniques   \cite{Umari_2010,Giustino_2010} already known in TDDFT. \cite{Walker_2006}  Such techniques proved to be very efficient in the case in particular of planewave basis sets that generate very large number of empty states.

%%
Another approach, that proved very fruitful, lies in the so-called space-time approach by Rojas and coworkers, \cite{Rojas_1995} that borrows the idea of Laplace transform  formulation, already used in quantum chemistry perturbation theories, \cite{Almlof_1991,Haser_1992} with the further concept of separability 



\noindent {\textbf{The challenge of Analytic gradients.}} \\

An additional issue concerns the formalism taken to calculate the ground-state energy for a given atomic configuration. Since the BSE formalism presented so far concerns the calculation of the electronic excitations, namely the difference of energy between the GS and the ES, gradients of the ES absolute energy require 

This points to another direction for the BSE foramlism, namely the calculation of GS total energy with the correlation energy calculated at the BSE level. Such a task was performed by several groups using in particular the adiabatic connection fluctuation-dissipation theorem (ACFDT), focusing in particular on small dimers. \cite{Olsen_2014,Holzer_2018,Li_2020,Loos_2020}

\noindent {\textbf{The Triplet Instability Challenge.}} \\

The analysis of the singlet-triplet splitting is central to numerous applications such as singlet fission, thermally activated delayed fluorescence (TADF) or 
stability analysis of restricted closed-shell solutions at the HF \cite{Seeger_1977} and TD-DFT \cite{Bauernschmitt_1996} levels. 
contaminating as well TD-DFT calculations with popular range-separated hybrids (RSH) that generally contains a large fraction of exact exchange in the long-range. \cite{Sears_2011}
While TD-DFT with RSH can benefit from tuning the range-separation parameter as a mean to act on the triplet instability, \cite{Sears_2011} BSE calculations do not offer this pragmatic way-out since the screened Coulomb potential that builds the kernel does not offer any parameter  to tune.

benchmarks \cite{Jacquemin_2017b,Rangel_2017}

a first cure was offered by hybridizing TD-DFT and BSE, namely adding to the BSE kernel the correlation part of the underlying DFT functional used to build the susceptibility and resulting screened Coulomb potential $W$.  \cite{Holzer_2018}

\noindent {\textbf{Dynamical kernels and multiple excitations.}} \\

\cite{Zhang_2013}


\noindent {\textbf{Core-level spectroscopy.}}. \\
XANES, 
\cite{Olovsson_2009,Vinson_2011}


diabatization and conical intersections \cite{Kaczmarski_2010}



%%%%%%%%%%%%%%%%%%
%%% CONCLUSION %%%
%%%%%%%%%%%%%%%%%%
Here goes the conclusion.

%%%%%%%%%%%%%%%%%%%%%%%%
%%% ACKNOWLEDGEMENTS %%%
%%%%%%%%%%%%%%%%%%%%%%%%
DJ acknowledges the \textit{R\'egion des Pays de la Loire} for financial support. 

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%%% BIBLIOGRAPHY %%%
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