diff --git a/Manuscript/SRGGW.tex b/Manuscript/SRGGW.tex index 6d55f24..e03fbe6 100644 --- a/Manuscript/SRGGW.tex +++ b/Manuscript/SRGGW.tex @@ -246,10 +246,16 @@ Various choices for $\bSig^\qs$ are possible but the most popular is the followi \Sigma_{pq}^\qs = \frac{1}{2}\Re[\Sigma_{pq}(\epsilon_p) + \Sigma_{pq}(\epsilon_q) ]. \end{equation} which was first 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 functions by Ismail-Beigi. \cite{Ismail-Beigi_2017} +The corresponding matrix elements are +\begin{equation} + \label{eq:sym_qsGW} + \Sigma_{pq}^{\titou{\text{qs}}}(\eta) = \frac{1}{2} \sum_{r\nu} \qty( \frac{\Delta_{pr\nu}}{\Delta_{pr\nu}^2 + \titou{\eta^2}} +\frac{\Delta_{qr\nu}}{\Delta_{qr\nu}^2 + \titou{\eta^2}} ) W_ {p,r\nu} \titou{W_{q,r\nu}}. +\end{equation} +with $\Delta_{pr\nu} = \epsilon_p - \epsilon_r - \sgn(\epsilon_r-\epsilon_F)\Omega_\nu$ (where $\epsilon_F$ is the energy of the Fermi level). One of the main results of the present manuscript is the derivation, from first principles, of an alternative static Hermitian form for the $GW$ self-energy. Once again, in cases where multiple solutions have large spectral weights, self-consistency can be difficult to reach at the qs$GW$ level. -Multiple solutions of Eq.~\eqref{eq:G0W0} arise due to the $\omega$ dependence of the self-energy. +Multiple solutions of Eq.~\eqref{eq:G0W0} arise due to the $\omega$ dependence of th e self-energy. Therefore, by suppressing this dependence, the static approximation relies on the fact that there is well-defined quasiparticle solutions. If it is not the case, the qs$GW$ self-consistent scheme inevitably oscillates between solutions with large spectral weights. \cite{Forster_2021} @@ -512,9 +518,8 @@ Collecting every second-order term in the flow equation and performing the block which can be solved by simple integration along with the initial condition $\bF^{(2)}=\bO$ to give \begin{multline} F_{pq}^{(2)}(s) = \sum_{r\nu} \frac{\Delta_{pr\nu}+ \Delta_{qr\nu}}{\Delta_{pr\nu}^2 + \Delta_{qr\nu}^2} W_{p,r\nu} W^{\dagger}_{r\nu,q} \\ - \times \qty[1 - e^{-(\Delta_{pr\nu}^2 + \Delta_{qr\nu}^2) s}], + \times \qty[1 - e^{-(\Delta_{pr\nu}^2 + \Delta_{qr\nu}^2) s}]. \end{multline} -with $\Delta_{pr\nu} = \epsilon_p - \epsilon_r - \sgn(\epsilon_r-\epsilon_F)\Omega_\nu$ (where $\epsilon_F$ is the energy of the Fermi level). At $s=0$, the second-order correction vanishes while, for $s\to\infty$, it tends towards the following static limit \begin{equation} @@ -529,38 +534,34 @@ This transformation is done gradually starting from the states that have the lar \subsection{Alternative form of the static self-energy} % ///////////////////////////% -\PFL{This part has to be rewritten because it is too confusing.} -Interestingly, the static limit, \ie the $s\to\infty$ limit of Eq.~\eqref{eq:GW_renorm}, defines an alternative qs$GW$ approximation to the one defined by Eq.~\eqref{eq:sym_qsgw} with matrix elements -\begin{equation} - \label{eq:sym_qsGW} - \Sigma_{pq}^{\titou{\text{qs}}}(\eta) = \frac{1}{2} \sum_{r\nu} \qty( \frac{\Delta_{pr\nu}}{\Delta_{pr\nu}^2 + \titou{\eta^2}} +\frac{\Delta_{qr\nu}}{\Delta_{qr\nu}^2 + \titou{\eta^2}} ) W_{p,r\nu} \titou{W_{q,r\nu}}. -\end{equation} -This alternative static form will be referred to as SRG-qs$GW$ in the following. -Both approximations are closely related as they share the same diagonal terms when $\eta=0$. -Also, note that the SRG static approximation is naturally Hermitian as opposed to the usual case where it is enforced by symmetrization. - -However, as we shall discuss further in Sec.~\ref{sec:results}, the convergence of the qs$GW$ scheme using \titou{$\widetilde{\bF}(\infty)$} is very poor. -This is similar to the symmetric case when the imaginary shift $\ii \eta$ is set to zero. -Indeed, in traditional qs$GW$ calculation increasing $\eta$ to ensure convergence in difficult cases is most often unavoidable. -Therefore, we will define the SRG-qs$GW$ static effective Hamiltonian as +Because the $s\to\infty$ limit of Eq.~(\ref{eq:GW_renorm}) is purely static, it can be seen as a qs$GW$ calculation with an alternative static approximation than the usual one of Eq.~(\ref{eq:sym_qsgw}). +Unfortunately, as we shall discuss further in Sec.~\ref{sec:results}, in the $s\to\infty$ limit self-consistently solving the renormalized quasi-particle equation is once again quite difficult, if not impossible. +However, one can define a more flexible new static self-energy, which will be referred to as SRG-qs$GW$ in the following, by discarding the dynamic part in Eq.~(\ref{eq:GW_renorm}). +This yields a $s$-dependent static self-energy which matrix elements read \begin{multline} \label{eq:SRG_qsGW} \Sigma_{pq}^{\text{SRG}}(s) = \sum_{r\nu} \frac{\Delta_{pr\nu}+ \Delta_{qr\nu}}{\Delta_{pr\nu}^2 + \Delta_{qr\nu}^2} W_{p,r\nu} W_{q,r\nu} \\ \times \qty[(1 - e^{-(\Delta_{pr\nu}^2 + \Delta_{qr\nu}^2) s} ], \end{multline} -which depends on one regularizing parameter $s$ analogously to $\eta$ in the usual qs$GW$. +Note that the SRG static approximation is naturally Hermitian as opposed to the usual case [see Eq.~(\ref{eq:sym_qsGW})] where it is enforced by symmetrization. +Yet, these approximations are closely related because for $\eta=0$ and $s\to\infty$ they share the same diagonal terms. + +It is well-known that in traditional qs$GW$ calculation increasing the imaginary shift $\ii\eta$ to ensure convergence in difficult cases is most often unavoidable. +Similarly, in SRG-qs$GW$ one might need to decrease the value of $s$ to ensure convergence. The fact that the $s\to\infty$ static limit does not always converge when used in a qs$GW$ calculation could have been predicted because in this limit even the intruder states have been included in $\tilde{\bF}$. Therefore, one should use a value of $s$ large enough to include almost every state but small enough to avoid intruder states. +The convergence properties and the accuracy of both static approximations will be quantitatively gauged in the results section. +In addition, -To conclude this section, we will discuss the case of discontinuities. -Indeed, previously we mentioned that intruder states are responsible for both the poor convergence of qs$GW$ and discontinuities in physical quantities at the $\GOWO$ level. +To conclude this section, the case of discontinuities will be briefly discussed. +Indeed, it has been previously mentioned that intruder states are responsible for both the poor convergence of qs$GW$ and discontinuities in physical quantities at the $\GOWO$ level. So is it possible to use the SRG machinery developed above to remove discontinuities? -In fact, not directly because discontinuities are due to intruder states in the dynamic part while we have seen just above that a finite value of $s$ is well-designed to avoid the intruder states in the static part. +Not directly because discontinuities are due to intruder states in the dynamic part while we have seen just above that a finite value of $s$ is well-designed to avoid the intruder states in the static part. However, doing a change of variable such that \begin{align} e^{-s} &= 1-e^{-t} & 1 - e^{-s} &= e^{-t} \end{align} -hence choosing a finite value of $t$ is well-designed to avoid discontinuities in the dynamic. -In fact, the dynamic part after the change of variable is closely related to the SRG-inspired regularizer introduced by Monino and Loos in Ref.~\onlinecite{Monino_2022}. +will reverse the situation and now a finite value of $t$ will be well-designed to avoid discontinuities in the renormalized dynamic part. +The dynamic part after the change of variable is actually closely related to the SRG-inspired regularizer introduced by Monino and Loos in Ref.~\onlinecite{Monino_2022}. %=================================================================% \section{Computational details}