ok with Sec II
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@ -212,24 +212,24 @@ and where
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are bare two-electron integrals in the spin-orbital basis.
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are bare two-electron integrals in the spin-orbital basis.
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The diagonal matrix $\boldsymbol{\Omega}$ contains the positive eigenvalues of the RPA problem defined in Eq.~\eqref{eq:full_dRPA} and its elements $\Omega_\nu$ appear in Eq.~\eqref{eq:GW_selfenergy}.
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The diagonal matrix $\boldsymbol{\Omega}$ contains the positive eigenvalues of the RPA problem defined in Eq.~\eqref{eq:full_dRPA} and its elements $\Omega_\nu$ appear in Eq.~\eqref{eq:GW_selfenergy}.
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In the Tamm-Dancoff approximation (TDA), one sets $\bB = \bO$ in Eq.~\eqref{eq:full_dRPA} which reduces to a Hermitian eigenvalue problem of the form $\bA \bX = \bX \bOm$ (hence $\bY=0$).
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%In the Tamm-Dancoff approximation (TDA), one sets $\bB = \bO$ in Eq.~\eqref{eq:full_dRPA} which reduces to a Hermitian eigenvalue problem of the form $\bA \bX = \bX \bOm$ (hence $\bY=0$).
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As mentioned above, because of the frequency dependence of the self-energy, solving exactly the quasiparticle equation \eqref{eq:quasipart_eq} is a rather complicated task.
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As mentioned above, because of the frequency dependence of the self-energy, solving exactly the quasiparticle equation \eqref{eq:quasipart_eq} is a rather complicated task.
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Hence, several approximate schemes have been developed to bypass full self-consistency.
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Hence, several approximate schemes have been developed to bypass full self-consistency.
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The most popular strategy is the one-shot (perturbative) $G_0W_0$ scheme, where the self-consistency is completely abandoned, and the off-diagonal elements of Eq.~\eqref{eq:quasipart_eq} are neglected.
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The most popular strategy is the one-shot (perturbative) $GW$ scheme, $G_0W_0$, where the self-consistency is completely abandoned, and the off-diagonal elements of Eq.~\eqref{eq:quasipart_eq} are neglected.
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Assuming a HF starting point, this results in $K$ quasiparticle equations that read
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Assuming a HF starting point, this results in $K$ quasiparticle equations that read
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\begin{equation}
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\begin{equation}
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\label{eq:G0W0}
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\label{eq:G0W0}
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\epsilon_p^{\HF} + \Sigma_{pp}(\omega) - \omega = 0,
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\epsilon_p^{\HF} + \Sigma_{pp}(\omega) - \omega = 0,
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\end{equation}
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\end{equation}
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where $\Sigma_{pp}(\omega)$ are the diagonal elements of $\bSig$ and $\epsilon_p^{\HF}$ are the HF orbital energies.
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where $\Sigma_{pp}(\omega)$ are the diagonal elements of $\bSig$ and $\epsilon_p^{\HF}$ are the HF orbital energies.
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The previous equations are non-linear with respect to $\omega$ and therefore have multiple solutions $\epsilon_{p,s}$ for a given $p$ (where the index $s$ is numbering solutions).
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The previous equations are non-linear with respect to $\omega$ and therefore have multiple solutions $\epsilon_{p,z}$ for a given $p$ (where the index $z$ is numbering solutions).
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These solutions can be characterized by their spectral weight given by the renormalization factor
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These solutions can be characterized by their spectral weight given by the renormalization factor
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\begin{equation}
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\begin{equation}
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\label{eq:renorm_factor}
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\label{eq:renorm_factor}
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0 \leq Z_{p,s} = \qty[ 1 - \eval{\pdv{\Sigma_{pp}(\omega)}{\omega}}_{\omega=\epsilon_{p,s}} ]^{-1} \leq 1.
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0 \leq Z_{p,z} = \qty[ 1 - \eval{\pdv{\Sigma_{pp}(\omega)}{\omega}}_{\omega=\epsilon_{p,z}} ]^{-1} \leq 1.
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\end{equation}
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\end{equation}
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The solution with the largest weight is referred to as the quasiparticle while the others are known as satellites (or shake-up transitions).
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The solution with the largest weight $Z_p \equiv Z_{p,z=0}$ is referred to as the quasiparticle while the others are known as satellites (or shake-up transitions).
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However, in some cases, Eq.~\eqref{eq:G0W0} can have two (or more) solutions with similar weights, hence the quasiparticle is not well-defined.
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However, in some cases, Eq.~\eqref{eq:G0W0} can have two (or more) solutions with similar weights, hence the quasiparticle is not well-defined.
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One obvious drawback of the one-shot scheme mentioned above is its starting point dependence.
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One obvious drawback of the one-shot scheme mentioned above is its starting point dependence.
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@ -264,7 +264,7 @@ Multiple solutions of Eq.~\eqref{eq:G0W0} arise due to the $\omega$ dependence o
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Therefore, by suppressing this dependence, the static approximation relies on the fact that there is well-defined quasiparticle solutions.
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Therefore, by suppressing this dependence, the static approximation relies on the fact that there is well-defined quasiparticle solutions.
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If it is not the case, the self-consistent qs$GW$ scheme inevitably oscillates between solutions with large spectral weights. \cite{Forster_2021}
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If it is not the case, the self-consistent qs$GW$ scheme inevitably oscillates between solutions with large spectral weights. \cite{Forster_2021}
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The satellites causing convergence problems are the above-mentioned intruder states. \cite{Monino_2022}
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The satellites causing convergence issues are the above-mentioned intruder states. \cite{Monino_2022}
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One can deal with them by introducing \textit{ad hoc} regularizers.
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One can deal with them by introducing \textit{ad hoc} regularizers.
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For example, the $\ii\eta$ term in the denominators of Eq.~\eqref{eq:GW_selfenergy}, sometimes referred to as a broadening parameter linked to the width of the quasiparticle peak, is similar to the usual imaginary-shift regularizer employed in various other theories plagued by the intruder-state problem. \cite{Surjan_1996,Forsberg_1997,Monino_2022,Battaglia_2022}.
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For example, the $\ii\eta$ term in the denominators of Eq.~\eqref{eq:GW_selfenergy}, sometimes referred to as a broadening parameter linked to the width of the quasiparticle peak, is similar to the usual imaginary-shift regularizer employed in various other theories plagued by the intruder-state problem. \cite{Surjan_1996,Forsberg_1997,Monino_2022,Battaglia_2022}.
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