diff --git a/BSE-PES.tex b/BSE-PES.tex index e4c2da5..85e3c6b 100644 --- a/BSE-PES.tex +++ b/BSE-PES.tex @@ -313,10 +313,11 @@ With the Mulliken notation for the bare two-electron integrals \end{equation} the $(\bA{\IS},\bB{\IS})$ BSE matrix elements read: \begin{subequations} -\label{eq:LR_BSE} \begin{align} + \label{eq:LR_BSE-A} \ABSE{ia,jb}{\IS} & = \delta_{ij} \delta_{ab} (\eGW{a} - \eGW{i}) + \IS \qty[ 2 \ERI{ia}{jb} - \W{ij,ab}{\IS} ], \\ + \label{eq:LR_BSE-B} \BBSE{ia,jb}{\IS} & = \lambda \qty[ 2 \ERI{ia}{bj} - \W{ib,aj}{\IS} ], \end{align} \end{subequations} @@ -345,10 +346,11 @@ where the spectral weights at coupling strength $\lambda$ read In Eq.~\eqref{eq:W}, $\eta$ is a positive infinitesimal, and $\OmRPA{m}{\IS}$ are the direct (\ie, without exchange) RPA neutral excitation energies computed by solving the linear eigenvalue problem \eqref{eq:LR} with the following matrix elements \begin{subequations} -\label{eq:LR_RPA} \begin{align} + \label{eq:LR_RPA-A} \ARPA{ia,jb}{\IS} & = \delta_{ij} \delta_{ab} (\eHF{a} - \eHF{i}) + 2 \IS \ERI{ia}{jb}, \\ + \label{eq:LR_RPA-B} \BRPA{ia,jb}{\IS} & = 2 \IS \ERI{ia}{bj}, \end{align} \end{subequations} @@ -356,12 +358,13 @@ where $\eHF{p}$ are the HF orbital energies. The relation between the BSE formalism and the well-known RPAx approach can be obtained by switching off the screening %namely setting $\epsilon_{\lambda}({\bf r},{\bf r}'; \omega) = \delta({\bf r}-{\bf r}')$ -so that $W^{\lambda}$ reduces to the bare Coulomb potential. In that limit, the $GW$ quasiparticle energies reduce to the Hartree-Fock eigenvalues, and Eqs.~\ref{eq:LR_BSE} to the RPAx equations: +so that $W^{\lambda}$ reduces to the bare Coulomb potential. In that limit, the $GW$ quasiparticle energies reduce to the Hartree-Fock eigenvalues, and Eqs.~\eqref{eq:LR_BSE-A} and \eqref{eq:LR_BSE-B} to the RPAx equations: \begin{subequations} -\label{eq:LR_RPAx} \begin{align} + \label{eq:LR_RPAx-A} \ARPAx{ia,jb}{\IS} & = \delta_{ij} \delta_{ab} (\eHF{a} - \eHF{i}) + \IS \qty[ 2 \ERI{ia}{jb} - \ERI{ij}{ab} ], \\ + \label{eq:LR_RPAx-B} \BRPAx{ia,jb}{\IS} & = \IS \qty[ 2 \ERI{ia}{bj} - \ERI{ib}{aj} ]. \end{align} \end{subequations} @@ -412,9 +415,9 @@ Therefore, an additional contribution to Eq.~\eqref{eq:EcBSE} originating from t However, as commonly done within RPA and RPAx (\ie, RPA with exchange), \cite{Toulouse_2009, Toulouse_2010, Holzer_2018} we shall neglect it in the present study. Equation \eqref{eq:EcBSE} can also be straightforwardly applied to RPA and RPAx, the only difference being the expressions of $\bA{\IS}$ and $\bB{\IS}$ used to obtain the eigenvectors $\bX{\IS}$ and $\bY{\IS}$ entering the definition of $\bP{\IS}$ [see Eq.~\eqref{eq:2DM}]. -For RPA, these expressions have been provided in Eq.~\eqref{eq:LR_RPA}, and their RPAx analogs in Eq.~\eqref{eq:LR_RPAx}. +For RPA, these expressions have been provided in Eqs.~\eqref{eq:LR_RPA-A} and \eqref{eq:LR_RPA-B}, and their RPAx analogs in Eqs.~\eqref{eq:LR_RPAx-A} and \eqref{eq:LR_RPAx-B}. In the following, we will refer to these two types of calculations as RPA@HF and RPAx@HF, respectively. -Finally, we will also consider the RPA@$GW$@HF scheme which consists in replacing the HF orbital energies in Eq.~\eqref{eq:LR_RPA} by the $GW$ quasiparticles energies. +Finally, we will also consider the RPA@$GW$@HF scheme which consists in replacing the HF orbital energies in Eq.~\eqref{eq:LR_RPA-A} by the $GW$ quasiparticles energies. Several important comments are in order here. For spin-restricted closed-shell molecular systems around their equilibrium geometry (such as the ones studied here), it is rare to encounter singlet instabilities as these systems can be classified as weakly correlated.