abstract

Manuscript/ExPerspective.tex

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-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.
+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 for small- and medium-size molecules.
+First, we describe the evolution of \textit{ab initio} state-of-the-art methods, with originally Roos' CASPT2 method, and then third-order coupled cluster methods as in the renowned Thiel set of excitation energies described in a remarkable series of papers in the 2000's.
+More recently, this quest for highly accurate excitation energies was reinitiated thanks to the resurgence of selected configuration interaction (SCI) methods and their efficient parallel implementation.
+These methods have been able to routinely deliver highly-accurate excitation energies for small molecules as well as medium-size molecules in small basis sets for single and double excitations.
+Second, we describe how these high-level methods and the creation of large, broad and accurate benchmark sets of excitation energies have allowed to assess fairly and accurately the performances of lower-order methods (\eg, TD-DFT, BSE, ADC, EOM-CC, etc) for different types of excited states.
+We conclude this \textit{Perspective} by discussing the current potentiality of these methods from both an expert and a non-expert point of view, and what we believe could be the future theoretical and technological developments in the field.
 \end{abstract}
 
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@@ -95,7 +93,7 @@ The factors that makes this quest for high accuracy particularly delicate are ve
 
 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 geometric relaxation and zero-point vibrational energy motion.
-Even more problematic, experimental spectra might not be available in gas phase, and, in the worst-case scenario, wrong assignments may have occured.
+Even more problematic, experimental spectra might not be available in gas phase, and, in the worst-case scenario, wrong assignments may have occurred.
 For a more faithful comparison between theory and experiment, although more computationally demanding, the so-called 0-0 energies are definitely a safer playground. \cite{Loo19b}
 %However, they require, from a theoretical point of view, access to the optimised excited-state geometry as well as its harmonic vibration frequencies.