abstract

Revision/ExPerspective.tex

@@ -167,11 +167,11 @@
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 \begin{abstract}
-\hl{We provide an overview of the steps that made possible to obtain increasingly accurate excitation energies and properties with computational chemistry tools, eventually leading to chemically accurate vertical transition 
-energies for small- and medium-size molecules. First, we describe the evolution of \textit{ab initio} methods employed to define benchmark values, with originally Roos' CASPT2 method, next CC3 methods as in 
-the renowned Thiel set, and more recently the resurgence of selected configuration interaction methods. These methods have been able to deliver highly accurate excitation energies for small molecules, as well as 
-medium-size molecules with compact basis sets, for both single and double excitations. Second, we describe how these high-level methods and the creation of diverse benchmark sets of excitation energies have allowed to 
-assess fairly the performance of computationally lighter theoretical models. We conclude by discussing  what we believe could be the future theoretical and technological developments in the field.}
+\hl{We provide an overview of the successive steps that made possible to obtain increasingly accurate excitation energies with computational chemistry tools, eventually leading to chemically accurate vertical transition energies for small- and medium-size molecules.     
+First, we describe the evolution of \textit{ab initio} methods employed to define benchmark values, with originally Roos' CASPT2 method, then the CC3 method as in the renowned Thiel set, and more recently the resurgence of selected configuration interaction methods. 
+The latter method has been able to deliver consistently, for both single and double excitations, highly accurate excitation energies for small molecules, as well as medium-size molecules with compact basis sets.
+Second, we describe how these high-level methods and the creation of representative benchmark sets of excitation energies have allowed to assess fairly and accurately the performance of computationally lighter methods. 
+We conclude by discussing the future theoretical and technological developments in the field.}
 \end{abstract}
 
 
@@ -289,7 +289,7 @@ The advent of time-dependent density-functional theory (TD-DFT) \cite{Run84,Cas9
 For low-lying valence excited states, TD-DFT calculations relying on hybrid exchange-correlation functionals have a typical error of $0.2$--$0.4$ eV.
 However, a large number of shortcomings were quickly discovered. \cite{Toz98,Toz99,Dre04,Mai04,Dre05,Lev06,Eli11}
 In the present context, one of the most annoying feature of TD-DFT --- in its most standard (adiabatic) approximation --- is its inability to describe, even qualitatively, charge-transfer states, \cite{Toz99,Dre04} Rydberg states, \cite{Toz98} and double excitations. \cite{Mai04,Lev06,Eli11}
-\hl{These issues, as well as other shortcomings of DFT and TD-DFT, have been related to the so-called delocalization error.} \cite{Aut14a}
+\hl{These issues, as well as other well-documented shortcomings of DFT and TD-DFT, are related to the so-called delocalization error.} \cite{Aut14a}
 One closely related issue is the selection of the exchange-correlation functional from an ever-growing zoo of functionals and the variation of the excitation energies that one can observe with different functionals. \cite{Goe19,Sue19} 
 More specifically, despite the development of new, more robust approaches (including the so-called range-separated \cite{Sav96,IIk01,Yan04,Vyd06} and double \cite{Goe10a,Bre16,Sch17} hybrids), it is still difficult (not to say impossible) to select a functional adequate for all families of transitions. \cite{Lau13} 
 Moreover, the difficulty of making TD-DFT systematically improvable obviously hampers its applicability.
@@ -462,7 +462,7 @@ Consequently, one can find in the literature several sets of excited-state geome
 There are also investigations of the accuracy of the nuclear gradients at the Franck-Condon point. \cite{Taj18,Taj19} 
 The interested reader may find useful several investigations reporting sets of reference oscillator strengths. \cite{Sil10c,Har14,Kan14,Loo18a,Loo20b} 
 \hl{Up to now, these investigations focusing on geometries and oscillator strengths have been mostly based on theory-vs-theory comparisons. Indeed, while for small compounds (\ie, typically from di- to tetra-atomic molecules), 
-one can find very accurate experimental measurements (excited state dipole moments, oscillator strengths, vibrational frequencies, etc), these data are usually not accessible in larger compounds. 
+one can find very accurate experimental measurements (excited state dipole moments, oscillator strengths, vibrational frequencies, etc), these data are usually not accessible for larger compounds. 
 Nevertheless, the emergence of X-ray free electron lasers might soon allow to obtain accurate experimental excited state densities and geometrical structures through diffraction experiments. 
 Such new experimental developments will likely offer new opportunities for experiment-vs-theory comparisons going beyond standard energetics.}
 Finally, more complex properties, such as two-photon cross-sections and vibrations, have been mostly determined at lower levels of theory, hinting at future studies on this particular subject.