In its press release announcing the attribution of the 2013 Nobel prize in Chemistry to Karplus, Levitt and 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 neutral electronic excitations in condensed matter systems, from molecules to extended solids, has witnessed the development of a large number of such approximate methods with numerous applications to a large variety of fields, from the prediction of the colour of precious metals and stones for jewellery, to the understanding e.g. of the basic principles behind photovoltaics, photocatalysis or DNA damage under irradiation in the context of biology. The present Perspective aims at describing the current status and upcoming challenges for the Bethe-Salpeter equation (BSE) formalism that, while sharing many features with time-dependent density functional theory (TD-DFT), including computational cost scaling with system size, relies on a different formalism, with specific difficulties but also potential solutions to known difficulties.
The Bethe-Salpeter equation formalism belongs to the family of Green's function many-body perturbation theories (MBPT). While the density and density matrix stand as the basic variables in DFT and Hartree-Fock, Green's function MBPT take the one-body and two-body Green's function as central quantities. The (time-ordered) one-body Green's function reads:
\begin{equation}
G(xt,x't') = -i \langle N | T {\hat\psi}(xt) {\hat\psi}^{\dagger}(x't')} | N \rangle
\end{equation}
where $| N \rangle$ is the N-electron ground-state wavefunction. The operators ${\hat\psi}(xt)$ and ${\hat\psi}^{\dagger}(x't')}$ remove/add an electron in space-spin-time positions (xt) and (x't'), while $T$ is the time-ordering operator. For (t>t') the one-body Green's function provides the amplitude of probability of finding
A central property of the one-body Green's function is that its spectral representation presents poles at the charged excitation of the system :
where $\varepsilon_s = E_s(N+1)- E_0(N)$ for $\varepsilon_s > \mu$ ($\mu$ chemical potential, $\eta$ small positive infinitesimal) and $\varepsilon_s = E_0(N)- E_s(N-1)$ for $\varepsilon_s < \mu$. The quantities $E_s(N+1)$ and $E_s(N-1)$ are the total energy of the s-th excited state of the (N+1) and (N-1)-electron systems, while $E_0(N)$ is the N-electron ground-state energy. Contrary to the Kohn-Sham eigenvalues, the Green's function poles $\varepsilon_s$ are the proper charging energies of the N-electron system, leading to well-defined ionization potentials and electronic affinities. Contrary to standard $\Delta$SCF techniques, the knowledge of $G$ provides the full ionization spectrum, as measured by direct and inverse photoemission, not only that associated with frontier orbtials. The $\labrace f_s \rbrace$ are called the Lehmann amplitudes.
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.
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.
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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}
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}
