ISTPC/2021/Lecture_2/ISTPC_Loos_2.tex

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% methods
\newcommand{\evGW}{evGW}
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\newcommand{\GOWO}{G$_0$W$_0$}
\newcommand{\GOW}{G$_0$W}
\newcommand{\GWO}{GW$_0$}
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\newcommand{\GOWOSOSEX}{{\GOWO}+SOSEX}
\newcommand{\GWSOSEX}{{\GW}+SOSEX}
\newcommand{\GnWn}[1]{G$_{#1}$W$_{#1}$}
\newcommand{\GOF}{G$_0$F2}
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% operators
\newcommand{\hH}{\Hat{H}}
% energies
\newcommand{\Ec}{E_\text{c}}
\newcommand{\EHF}{E_\text{HF}}
\newcommand{\EcK}{E_\text{c}^\text{Klein}}
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% orbital energies
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\newcommand{\eLUMO}{\epsilon_\text{LUMO}}
\newcommand{\cHF}[1]{c^\text{HF}_{#1}}
\newcommand{\cKS}[1]{c^\text{KS}_{#1}}
% Matrix elements
\newcommand{\A}[1]{A_{#1}}
\newcommand{\B}[1]{B_{#1}}
\renewcommand{\S}[1]{S_{#1}}
\newcommand{\ABSE}[1]{A^\text{BSE}_{#1}}
\newcommand{\BBSE}[1]{B^\text{BSE}_{#1}}
\newcommand{\ARPA}[1]{A^\text{RPA}_{#1}}
\newcommand{\BRPA}[1]{B^\text{RPA}_{#1}}
\newcommand{\dABSE}[1]{\delta A^\text{BSE}_{#1}}
\newcommand{\dBBSE}[1]{\delta B^\text{BSE}_{#1}}
\newcommand{\G}[1]{G_{#1}}
\newcommand{\Po}[1]{P_{#1}}
\newcommand{\W}[1]{W_{#1}}
\newcommand{\Wc}[1]{W^\text{c}_{#1}}
\newcommand{\vc}[1]{v_{#1}}
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\newcommand{\SigGWSOSEX}[1]{\Sigma^\text{\GWSOSEX}_{#1}}
\newcommand{\SigGF}[1]{\Sigma^\text{\GF}_{#1}}
\newcommand{\Z}[1]{Z_{#1}}
% excitation energies
\newcommand{\OmRPA}[1]{\Omega^\text{RPA}_{#1}}
\newcommand{\OmCIS}[1]{\Omega^\text{CIS}_{#1}}
\newcommand{\OmTDHF}[1]{\Omega^\text{TDHF}_{#1}}
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% Wigner symbols
\newcommand{\WJ}[3]{
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#1 & #2 & #3 \\
0 & 0 & 0 \\
\end{pmatrix}
}
\newcommand{\ERI}[3]{\qty(#1 #2 #3)}
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% Matrices
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% * HEAD DATA *
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\title[$GW$/BSE methods in chemistry]{
$GW$/BSE methods in chemistry:
Computational aspects
}
\author[PF Loos]{Pierre-Fran\c{c}ois LOOS}
\date{Online ISTPC 2021 school --- April 27th, 2021}
\institute[CNRS@LCPQ]{
Laboratoire de Chimie et Physique Quantiques (UMR 5626),\\
Universit\'e de Toulouse, CNRS, UPS, Toulouse, France.
}
\titlegraphic{
\includegraphics[width=0.3\textwidth]{fig/jarvis}
\\
\vspace{0.05\textheight}
\includegraphics[height=0.05\textwidth]{fig/UPS}
\hspace{0.2\textwidth}
\includegraphics[height=0.05\textwidth]{fig/ERC}
\hspace{0.2\textwidth}
\includegraphics[height=0.05\textwidth]{fig/LCPQ}
\hspace{0.2\textwidth}
\includegraphics[height=0.05\textwidth]{fig/CNRS}
}
\begin{document}
%%% SLIDE 1 %%%
\begin{frame}
\titlepage
\end{frame}
%
%%% SLIDE 2 %%%
\begin{frame}{Today's program}
\begin{itemize}
\item
\end{itemize}
\end{frame}
%
%-----------------------------------------------------
\section{Theory}
%-----------------------------------------------------
\subsection{Hedin's pentagon}
%-----------------------------------------------------
\begin{frame}{Hedin's pentagon}
\begin{columns}
\begin{column}{0.4\textwidth}
\centering
\includegraphics[width=0.8\linewidth]{fig/pentagon}
\\
\pub{Hedin, Phys Rev 139 (1965) A796}
\end{column}
\begin{column}{0.6\textwidth}
\begin{block}{What can you calculate with GW?}
\begin{itemize}
\item Ionization potentials (IP) given by occupied MO energies
\bigskip
\item Electron affinities (EA) given by virtual MO energies
\bigskip
\item HOMO-LUMO gap (or band gap in solids)
\bigskip
\item Singlet and triplet neutral excitations (vertical absorption energies)
\bigskip
\item Correlation and total energies
\end{itemize}
\end{block}
\end{column}
\end{columns}
\end{frame}
%-----------------------------------------------------
\subsection{GW flavours}
%-----------------------------------------------------
\begin{frame}{GW flavours}
\begin{block}{Acronyms}
\begin{itemize}
\bigskip
\item perturbative GW one-shot GW, or \green{\GOWO}
\bigskip
\item \orange{\evGW} or eigenvalue-only (partially) self-consistent GW
\bigskip
\item \red{\qsGW} or quasiparticle (partially) self-consistent GW
\bigskip
\item \violet{\scGW} or (fully) self-consistent GW
\bigskip
\item \purple{BSE} or Bethe-Salpeter equation for neutral excitations
\bigskip
\end{itemize}
\end{block}
\end{frame}
%-----------------------------------------------------
\subsection{Literature}
%-----------------------------------------------------
\begin{frame}{useful papers for chemists}
\begin{itemize}
\item \red{molGW:} Bruneval et al. Comp. Phys. Comm. 208 (2016) 149
\bigskip
\item \green{Turbomole:} van Setten et al. JCTC 9 (2013) 232; Kaplan et al. JCTC 12 (2016) 2528
\bigskip
\item \violet{Fiesta:} Blase et al. Chem. Soc. Rev. 47 (2018) 1022
\bigskip
\item \purple{FHI-AIMS:} Caruso et al. 86 (2012) 081102
\bigskip
\item \orange{Review:} Reining, WIREs Comput Mol Sci 2017, e1344. doi: 10.1002/wcms.1344; Onida et al. Rev. Mod. Phys. 74 (2002) 601.
\bigskip
\item \red{GW100:} Data set of 100 molecules. van Setten et al. JCTC 11 (2015) 5665
\end{itemize}
\end{frame}
%-----------------------------------------------------
\section{Implementation}
%-----------------------------------------------------
\subsection{\GOWO}
%-----------------------------------------------------
\begin{frame}{\GOWO}
\begin{block}{{\GOWO}~subroutine}
\begin{algorithmic}
\Procedure{Perturbative {\GW}}{}
\State Perform HF calculation to get $\beHF$ and $\bcHF$
\For{$p=1,\ldots,N$}
\State Compute \red{$\SigC{pp}(\omega)$} and \green{$\Z{p}(\omega)$}
\State $\eGOWO{p} = \eHF{p} + \green{\Z{p}(\eHF{p})} \Re[\red{\SigC{pp}(\eHF{p})}]$
\State \Comment{This is the linearized version of the}
\State \Comment{quasiparticle (QP) equation
$\omega = \eHF{p} + \Re[\red{\SigC{pp}(\omega)}]$}
\EndFor
\If{BSE}
\State Compute BSE excitations energies
\EndIf
\EndProcedure
\end{algorithmic}
\end{block}
\end{frame}
\begin{frame}{\GOWO}
\begin{block}{\red{Correlation part of the self-energy:}}
\begin{equation*}
\red{\SigC{pq}(\omega)}
= 2 \sum_{ix}\frac{\violet{[pi|x] [qi|x]}}{\omega - \eHF{i} + \orange{\Om{x}} - i \eta}
+ 2 \sum_{ax}\frac{\violet{[pa|x] [qa|x]}}{\omega - \eHF{a} - \orange{\Om{x}} + i \eta}
\end{equation*}
\end{block}
\begin{block}{\green{Renormalization factor}}
\begin{equation*}
\green{\Z{p}(\omega)} = \qty[ 1 - \pdv{\Re[\red{\SigGW{pp}(\omega)}]}{\omega} ]^{-1}
\end{equation*}
\end{block}
\begin{block}{\violet{Screened two-electron MO integrals}}
\begin{equation*}
\violet{[pq|x]} = \sum_{ia} (pq|ia) \orange{(\bX+\bY)_{ia}^{x}}
\end{equation*}
\end{block}
\begin{block}{\orange{RPA excitation energies}}
\small
\begin{equation*}
\begin{pmatrix}
\bA & \bB \\
\bB & \bA \\
\end{pmatrix}
\orange{\begin{pmatrix}
\bX \\
\bY \\
\end{pmatrix}}
=
\orange{\bOm}
\begin{pmatrix}
\bm{1} & 0 \\
0 & \bm{-1} \\
\end{pmatrix}
\orange{\begin{pmatrix}
\bX \\
\bY \\
\end{pmatrix}}
\end{equation*}
\begin{align*}
A^\text{RPA}_{ia,jb} & = \delta_{ij} \delta_{ab} (\epsilon_a - \epsilon_i) + 2 (ia|jb)
&
B^\text{RPA}_{ia,jb} & = 2 (ia|bj)
\end{align*}
\end{block}
\end{frame}
%-----------------------------------------------------
\subsection{\evGW}
%-----------------------------------------------------
\begin{frame}{\evGW}
\begin{block}{{\evGW} subroutine}
\begin{algorithmic}
\Procedure{Partially self-consistent {\evGW}}{}
\State Perform HF calculation to get $\beHF$ and $\bcHF$
\State Set $\beGnWn{-1} = \beHF$ and $n = 0$
\While{$\max{\abs{\bDelta}} < \tau$}
\For{$p=1,\ldots,N$}
\State Compute \red{$\SigC{pp}(\omega)$}
\State Solve $\omega = \eHF{p} + \Re[\red{\SigC{pp}(\omega)}]$ to obtain $\eGnWn{p}{n}$
\EndFor
\State $\bDelta = \beGnWn{n} - \beGnWn{n-1}$
\State $n \leftarrow n + 1$
\EndWhile
\If{BSE}
\State Compute BSE excitations energies
\EndIf
\EndProcedure
\end{algorithmic}
\end{block}
\end{frame}
%-----------------------------------------------------
\subsection{\qsGW}
%-----------------------------------------------------
\begin{frame}{\qsGW}
\begin{block}{{\qsGW} subroutine}
\begin{algorithmic}
\Procedure{Partially self-consistent {\qsGW}}{}
\State Perform HF calculation to get $\beHF$ and $\bcHF$
\State Set $\beGnWn{-1} = \beHF$, $\bcGnWn{-1} = \bcHF$ and $n = 0$
\While{$\max{\abs{\bDelta}} < \tau$}
\State Form \red{$\bSigC(\omega)$} and symmetrize it: $\red{\bSigC(\omega)} \leftarrow (\red{\bSigC(\omega)}^\dag + \red{\bSigC(\omega)})/2$
\State Form $\green{\bF(\omega)} = \bFHF + \red{\bSigC(\omega)}$
\State Diagonalize $\green{\bF(\beGnWn{n-1})}$ to get $\beGnWn{n}$ and $\bcGnWn{n}$
\State $\bDelta = \beGnWn{n} - \beGnWn{n-1}$
\State $n \leftarrow n + 1$
\EndWhile
\If{BSE}
\State Compute BSE excitations energies
\EndIf
\EndProcedure
\end{algorithmic}
\end{block}
\end{frame}
%-----------------------------------------------------
\subsection{BSE}
%-----------------------------------------------------
\begin{frame}{BSE}
\begin{block}{Bethe-Salpeter equation}
\begin{equation*}
\begin{pmatrix}
\bA & \bB \\
\bB & \bA \\
\end{pmatrix}
\purple{\begin{pmatrix}
\bX \\
\bY \\
\end{pmatrix}}
=
\purple{\bOm}
\begin{pmatrix}
\bm{1} & 0 \\
0 & \bm{-1} \\
\end{pmatrix}
\purple{\begin{pmatrix}
\bX \\
\bY \\
\end{pmatrix}}
\end{equation*}
\begin{equation*}
(\bA - \bB)^{1/2} (\bA + \bB) (\bA - \bB)^{1/2} \bZ = \bOm^2 \bZ,
\end{equation*}
\begin{equation*}
\bX + \bY = \bOm^{-1/2} (\bA - \bB)^{1/2} \bZ.
\end{equation*}
\begin{align*}
A^\text{BSE}_{ia,jb} & = A^\text{RPA}_{ia,jb} - (ij|ab) + 4 \sum_{x} \frac{[ij|x][ab|x]}{\Om{x}}
\\
B^\text{BSE}_{ia,jb} & = B^\text{RPA}_{ia,jb} - (ib|aj) + 4 \sum_{x} \frac{[ib|x][aj|x]}{\Om{x}}
\end{align*}
\end{block}
\end{frame}
%-----------------------------------------------------
\subsection{$\Ec$}
%-----------------------------------------------------
\begin{frame}{Correlation energy}
\begin{block}{RPA correlation energy or Klein functional}
\begin{equation*}
\label{eq:Ec-RPA}
\EcRPA = -\sum_{p} \qty(\ARPA{pp} - \Om{p})
\end{equation*}
\end{block}
\begin{block}{Galitskii-Migdal functional}
\begin{equation*}
\label{eq:GM}
\EcGM = \frac{-i}{2}\sum_{pq}^{\infty}\int \frac{d\omega}{2\pi} \SigC{pq}(\omega) \G{pq}(\omega) e^{i\omega\eta}
\end{equation*}
\end{block}
\end{frame}
\end{document}