diff --git a/Manuscript/SRGGW.tex b/Manuscript/SRGGW.tex index 2a75ec1..7a5c2a3 100644 --- a/Manuscript/SRGGW.tex +++ b/Manuscript/SRGGW.tex @@ -134,17 +134,17 @@ The goal of this manuscript is to determine if the SRG formalism can effectively The manuscript is organized as follows. We begin by reviewing the $GW$ approximation in Sec.~\ref{sec:gw} and then briefly review the SRG formalism in Sec.~\ref{sec:srg}. -Section~\ref{sec:theoretical} is concluded by a perturbative analysis of SRG applied to $GW$ (see Sec.~\ref{sec:srggw}). +A perturbative analysis of SRG applied to $GW$ is presented in Sec.~\ref{sec:srggw}. The computational details are provided in Sec.~\ref{sec:comp_det} before turning to the results section. This section starts by %=================================================================% -\section{Theoretical background} - \label{sec:theoretical} +%\section{Theoretical background} +% \label{sec:theoretical} %=================================================================% %%%%%%%%%%%%%%%%%%%%%% -\subsection{Many-body perturbation theory in the GW approximation} +\section{The $GW$ approximation} \label{sec:gw} %%%%%%%%%%%%%%%%%%%%%% @@ -183,22 +183,25 @@ In fact, these cases are related to the discontinuities and convergence problems One obvious flaw of the one-shot scheme mentioned above is its starting point dependence. Indeed, in Eq.~\eqref{eq:G0W0} we chose to use the HF orbital energies but this is arbitrary and one could have chosen Kohn-Sham orbitals for example. -Therefore, one could try to optimize the starting point to obtain the best one-shot energies possible. \cite{Korzdorfer_2012,Marom_2012,Bruneval_2013,Gallandi_2015,Caruso_2016, Gallandi_2016} -Alternatively, one could solve this equation self-consistently leading to the eigenvalue-only self-consistent scheme. \cite{Shishkin_2007,Blase_2011b,Marom_2012,Kaplan_2016,Wilhelm_2016} +Therefore, one can \titou{optimize} the starting point to obtain the best one-shot energies possible, which is commonly done. \cite{Korzdorfer_2012,Marom_2012,Bruneval_2013,Gallandi_2015,Caruso_2016, Gallandi_2016} +\PFL{Maybe it is worth mentioning here that is is a fairly heuristic approach that is obviously system dependent?} +Alternatively, one could solve this equation self-consistently leading to the eigenvalue-only self-consistent scheme (ev$GW$). \cite{Shishkin_2007,Blase_2011b,Marom_2012,Kaplan_2016,Wilhelm_2016} To do so the energy of the quasi-particle solution of the previous iteration is used to build Eq.~\eqref{eq:G0W0} and then this equation is solved for $\omega$ again until convergence is reached. -However, if the quasi-particle solution is not well-defined, self-consistency can be quite difficult, if not impossible, to reach. -Even if self-consistency has been reached, the starting point dependence has not been totally removed because the results still depend on the starting molecular orbitals. \cite{Marom_2012} +\PFL{This is not quite right. It is probably going to be easier to explain when you're going to introduce the explicit expressions of these quantities.} +However, if the quasi-particle solution is not well-defined, reaching self-consistency can be quite difficult, if not impossible. +Even at convergence, the starting point dependence is not totally removed as the results still depend on the starting orbitals. \cite{Marom_2012} -To update both the energies and the molecular orbitals, one needs to take into account the off-diagonal elements in Eq.~\eqref{eq:quasipart_eq}. +To update both energies and orbitals, one must take into account the off-diagonal elements in Eq.~\eqref{eq:quasipart_eq}. To take into account the effect of off-diagonal elements without fully solving the quasi-particle equation, one can resort to the quasi-particle self-consistent (qs) scheme in which $\bSig(\omega)$ is replaced by a static approximation $\bSig^{\qs}$. The algorithm to solve the qs problem is totally analog to the HF case with $\bF$ replaced by $\bF + \bSig^{\qs}$. Various choices for $\bSig^\qs$ are possible but the most popular one is the following Hermitian approximation \begin{equation} \label{eq:sym_qsgw} - \Sigma_{pq}^\qs = \frac{1}{2}\Re\left(\Sigma_{pq}(\epsilon_p) + \Sigma_{pq}(\epsilon_q) \right). + \Sigma_{pq}^\qs = \frac{1}{2}\Re[\Sigma_{pq}(\epsilon_p) + \Sigma_{pq}(\epsilon_q) ]. \end{equation} -This form has first been introduced by Faleev and co-workers \cite{Faleev_2004,vanSchilfgaarde_2006,Kotani_2007} before being derived as the effective Hamiltonian that minimizes the length of the gradient of the Klein functional for non-interacting Green's function. \cite{Ismail-Beigi_2017} -One of the main results of this manuscript is the derivation from first principles of an alternative static Hermitian form, this will be done in the next section. +This form has first been introduced by Faleev and co-workers \cite{Faleev_2004,vanSchilfgaarde_2006,Kotani_2007} before being derived as the \titou{form of the} effective Hamiltonian that minimizes the length of the gradient of the Klein functional for non-interacting Green's function. \cite{Ismail-Beigi_2017} +One of the main results of this manuscript is the derivation from first principles of an alternative static Hermitian form. +This will be done in the next section. In this case, as well self-consistency can be difficult to reach in cases where multiple solutions have large spectral weights. Multiple solutions arise due to the $\omega$ dependence of the self-energy. @@ -207,7 +210,7 @@ If it is not the case, the qs scheme will oscillate between the solutions with l Convergence problems arise when a shake-up configuration has an energy similar to the associated quasi-particle solution, this satellite is the so-called intruder state. The intruder state problem can be dealt with by introducing \textit{ad hoc} regularizers. -The $\ii eta$ term that is usually added in the denominator of the self-energy (see below) is the usual imaginary-shift regularizer used in various other theories flawed by intruder states. \cite{Battaglia_2022} \ant{more ref...} +The $\ii \eta$ term that is usually added in the denominator of the self-energy (see below) is the usual imaginary-shift regularizer used in various other theories flawed by intruder states. \cite{Battaglia_2022} \ant{more ref...} Various other regularizers are possible and in particular one of us has shown that a regularizer inspired by the SRG had some advantages, in the $GW$ case, over the imaginary shift one. \cite{Monino_2022} But it would be more rigorous, and more instructive, to obtain this regularizer from first principles by applying the SRG formalism to many-body perturbation theory. This is the aim of this work. @@ -294,7 +297,7 @@ As can be readily seen in Eq.~\eqref{eq:GWlin}, the blocks $V^\text{2h1p}$ and $ Therefore, these blocks will be the target of our SRG transformation but before going into more detail we will review the SRG formalism. %%%%%%%%%%%%%%%%%%%%%% -\subsection{The similarity renormalization group} +\section{The similarity renormalization group} \label{sec:srg} %%%%%%%%%%%%%%%%%%%%%% @@ -355,7 +358,7 @@ Hence, the generator $\boldsymbol{\eta}(s)$ admits a perturbation expansion as w Then, one can collect order by order the terms in Eq.~\eqref{eq:flowEquation} and solve analytically the low-order differential equations. %%%%%%%%%%%%%%%%%%%%%% -\subsection{Renormalized GW} +\section{Regularized $GW$ approximation} \label{sec:srggw} %%%%%%%%%%%%%%%%%%%%%% @@ -403,7 +406,7 @@ Once the analytical low-order perturbative expansions are known they can be inse In particular, in this manuscript, the focus will be on the second-order renormalized quasi-particle equation. %///////////////////////////% -\subsubsection{Zeroth-order matrix elements} +\subsection{Zeroth-order matrix elements} %///////////////////////////% There is only one zeroth order term in the right-hand side of the flow equation @@ -447,7 +450,7 @@ Therefore, the zeroth order Hamiltonian is \ie it is independent of $s$. %///////////////////////////% -\subsubsection{First-order matrix elements} +\subsection{First-order matrix elements} %///////////////////////////% Knowing that $\bHod{0}(s)=\bO$, the first order flow equation is @@ -470,7 +473,7 @@ Therefore, $W_{p,(q,v)}^{(1)}(s)$ are renormalized two-electrons screened integr Note the close similarity of the first-order element expressions with the ones of Evangelista in Ref.~\onlinecite{Evangelista_2014b} obtained in a second quantization formalism (see also Ref.~\onlinecite{Hergert_2016}). %///////////////////////////% -\subsubsection{Second-order matrix elements} +\subsection{Second-order matrix elements} % ///////////////////////////% The second-order renormalized quasi-particle equation is given by