minor corrections

Manuscript/ExPerspective.bib

@@ -1,8 +1,7 @@
 %% This BibTeX bibliography file was created using BibDesk.
 %% http://bibdesk.sourceforge.net/
 
-
+%% Created for Pierre-Francois Loos at 2019-11-19 20:03:52 +0100 
-%% Created for Denis Jacquemin at 2019-11-19 19:32:14 +0100 
 
 
 %% Saved with string encoding Unicode (UTF-8) 
@@ -83,6 +82,28 @@
 @string{theo = {J. Mol. Struct. (THEOCHEM)}}
 
 
+@article{Odd78,
+	Author = {J. Oddershede},
+	Date-Added = {2019-11-19 19:58:44 +0100},
+	Date-Modified = {2019-11-19 19:59:44 +0100},
+	Journal = {Adv. Quantum Chem.},
+	Pages = {275--352},
+	Title = {Polarization Propagator Calculations},
+	Volume = {11},
+	Year = {1978}}
+
+@article{Pac96,
+	Author = {M. J. Packer and E. K. Dalskov and T. Enevoldsen and H. J. A. Jensen and J. Oddershede},
+	Date-Added = {2019-11-19 19:55:57 +0100},
+	Date-Modified = {2019-11-19 20:03:41 +0100},
+	Doi = {10.1063/1.472430},
+	Journal = {J. Chem. Phys.},
+	Pages = {5886},
+	Title = {A new Implementation of the Second-Order Polarization Propagator Approximation (SOPPA): The Excitation Spectra of Benzene and Naphthalene},
+	Volume = {105},
+	Year = {1996},
+	Bdsk-Url-1 = {https://doi.org/10.1063/1.472430}}
+
 @article{Loo19b,
 	Author = {Loos, Pierre-Francois and Jacquemin, Denis},
 	Date-Added = {2019-11-19 19:26:28 +0100},

Manuscript/ExPerspective.tex

@@ -92,7 +92,7 @@ First of all (and maybe surprisingly), it is, in most cases, tricky to obtain re
 do not usually correspond to theoretical values as one needs to take into account both 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, no clear 
 assignment could be made. 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{Die04b,Win13,Fan14b,Loo19b}
 
-Second, developing theories suited for excited states is usually more complex than their ground-state equivalent as a variational principle may not be available for excited states.
+Second, developing theories suited for excited states is usually more complex and costly than their ground-state equivalent, as one might lack a proper variational principle for excited-state energies. 
 As a consequence, for a given level of theory, excited-state methods are usually less accurate than their ground-state counterpart.
 
 Another feature that makes excited states particularly fascinating and challenging is that they can be both extremely close in energy from each other and have very different natures ($\pi \ra \pi^*$, $n \ra \pi^*$, charge transfer, double excitation, valence, Rydberg, singlet, 
@@ -198,6 +198,7 @@ It is also noteworthy that CCSDT and CC3 are also able to detect the presence of
 %%% ADC METHODS %%%
 %%%%%%%%%%%%%%%%%%%
 It is also important to mention the recent rejuvenation of the second- and third-order algebraic diagrammatic construction [ADC(2) \cite{Sch82} and ADC(3) \cite{Tro99,Har14}] methods that scale as $N^5$ and $N^6$, respectively.
+These methods are related to the older second- and third-order polarization propagator approaches (SOPPA and TOPPA). \cite{Odd78,Pac96}
 This renaissance was certainly initiated by the enormous amount of work invested by Dreuw's group in order to provide a fast and efficient implementation of these methods, \cite{Dre15} including the analytical gradients, \cite{Reh19} as well as other interesting variants.
 These Green's function one-electron propagator techniques indeed represent valuable alternatives thanks to their reduced cost compared to their CC equivalents.
 In that regard, ADC(2) is particularly attractive with an error around $0.1$--$0.2$ eV.
@@ -341,7 +342,7 @@ It would likely stimulate further theoretical developments in excited-state meth
 Besides all the studies described above aiming at reaching chemically accurate vertical transition energies, it should be pointed out that an increasing amount of effort is currently devoted to the obtention of highly-trustable excited-state properties. 
 This includes, first, 0-0 energies, \cite{Die04b,Hat05c,Goe10a,Sen11b,Win13,Fan14b,Loo18b,Loo19a,Loo19b} which, as mentioned above, offer well-grounded comparisons with experiment. 
 However, because 0-0 energies are fairly insensitive to the underlying molecular geometries, \cite{Sen11b,Win13,Loo19a} they are not a good indicator of their overall quality. 
-Consequently, one can find in the literature several sets of excited-state geometries obtained at various levels of theory, \cite{Pag03,Gua13,Bou13,Tun16,Bud17} some of them are determined using state-of-the-art models. \cite{Gua13,Bud17} 
+Consequently, one can find in the literature several sets of excited-state geometries obtained at various levels of theory, \cite{Pag03,Gua13,Bou13,Tun16,Bud17} some of them being determined using state-of-the-art models. \cite{Gua13,Bud17} 
 There are also investigations of the accuracy of the nuclear gradients at the Franck-Condon point. \cite{Taj18,Taj19} 
 The interested researcher may find useful several investigations reporting sets of reference oscillator strengths. \cite{Sil10c,Har14,Kan14,Loo18a,Loo20b} 
 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.