diff --git a/Manuscript/FarDFT.tex b/Manuscript/FarDFT.tex index afe8570..b9f32ef 100644 --- a/Manuscript/FarDFT.tex +++ b/Manuscript/FarDFT.tex @@ -39,10 +39,11 @@ % operators \newcommand{\hHc}{\Hat{h}} \newcommand{\hT}{\Hat{T}} +\newcommand{\hWee}{\Hat{W}_\text{ee}} +\newcommand{\hGam}[1]{\Hat{\Gamma}^{#1}} \newcommand{\bH}{\boldsymbol{H}} \newcommand{\hVext}{\Hat{V}_\text{ext}} \newcommand{\vext}{v_\text{ext}} -\newcommand{\hWee}{\Hat{W}_\text{ee}} % functionals, potentials, densities, etc \newcommand{\F}[2]{F_{#1}^{#2}} @@ -85,9 +86,10 @@ \newcommand{\eK}[1]{K_{#1}} \newcommand{\eF}[2]{F_{#1}^{#2}} \newcommand{\ON}[2]{f_{#1}^{#2}} +\newcommand{\Det}[2]{\Phi_{#1}^{#2}} % Numbers -\newcommand{\Nens}{M} +\newcommand{\nEns}{M} \newcommand{\Nel}{N} \newcommand{\Norb}{K} @@ -187,9 +189,9 @@ Unless otherwise stated, atomic units are used throughout. \label{sec:theo} As mentioned above, eDFT for excited states is based on the GOK variational principle \cite{Gross_1988a} which states that a variational principle holds for the ensemble energy \begin{equation} - \E{}{\bw} = \sum_{I=0}^{\Nens-1} \ew{I} \E{}{(I)} + \E{}{\bw} = \sum_{I=0}^{\nEns-1} \ew{I} \E{}{(I)} \end{equation} -built from an ensemble of $\Nens$ electronic states with individual energies $\E{}{(0)} \le \ldots \le \E{}{(\Nens-1)}$, and (normalised) monotonically decreasing weights $\bw = (\ew{0},\ldots,\ew{M-1})$, \ie, $\sum_{I=0}^{\Nens-1} \ew{I} = 1$, and $\ew{0} \ge \ldots \ge \ew{\Nens-1}$. +built from an ensemble of $\nEns$ electronic states with individual energies $\E{}{(0)} \le \ldots \le \E{}{(\nEns-1)}$, and (normalised) monotonically decreasing weights $\bw = (\ew{0},\ldots,\ew{M-1})$, \ie, $\sum_{I=0}^{\nEns-1} \ew{I} = 1$, and $\ew{0} \ge \ldots \ge \ew{\nEns-1}$. Multiplet degeneracies can be easily handled by assigning the same weight to the degenerate states. One of the key feature of GOK-DFT in the present context is that one can easily extract individual excitation energies from the ensemble energy via differentiation with respect to individual weights: @@ -206,9 +208,15 @@ In GOK-DFT, one defines a universal (weight-dependent) ensemble functional $\F{} where $\vext(\br{})$ is the external potential. In the KS formulation of GOK-DFT, the universal ensemble functional (the weight-dependent analog of the Hohenberg-Kohn universal functional for ensembles) is decomposed as \begin{equation} - \F{}{\bw}[\n{}{}] = \Ts{\bw}[\n{}{}] + \E{\Hxc}{\bw}[\n{}{}], + \F{}{\bw}[\n{}{}] + = \Ts{\bw}[\n{}{}] + \E{\Hxc}{\bw}[\n{}{}] + = \Tr[ \hGam{\bw} \hT ] + \Tr[ \hGam{\bw} \hWee ], \end{equation} -where $\Ts{\bw}[\n{}{}]$ is the noninteracting ensemble kinetic energy functional and +where $\hT$ and $\hWee$ are the kinetic and electron-electron interaction potential operators, respectively, $\Ts{\bw}[\n{}{}]$ is the noninteracting ensemble kinetic energy functional, +\begin{equation} + \hGam{\bw} = \sum_{I=0}^{\nEns} \ew{I} \dyad{\Det{I}{\bw}} +\end{equation} +is the density matrix operator, $\Det{I}{\bw}$ are single-determinant wave functions (or configuration state functions) built with KS orbitals $\MO{p}{\bw}(\br{})$, and \begin{equation} \label{eq:exc_def} \begin{split} @@ -232,7 +240,7 @@ From the GOK-DFT ensemble energy expression in Eq.~\eqref{eq:Ew-GOK}, we obtain where \begin{align} \label{eq:nw} - \n{}{\bw}(\br{}) & = \sum_{I=0}^{\Nens-1} \ew{I} \n{}{(I)}(\br{}), + \n{}{\bw}(\br{}) & = \sum_{I=0}^{\nEns-1} \ew{I} \n{}{(I)}(\br{}), \\ \label{eq:nI} \n{}{(I)}(\br{}) & = \sum_{p}^{\Norb} \ON{p}{(I)} [\MO{p}{\bw}(\br{})]^2 @@ -267,7 +275,7 @@ Note that, although we have dropped the weight-dependency in the individual dens \section{Functional} \label{sec:func} The present work deals with the explicit construction of the (reduced) LDA xc functional $\e{\xc}{\bw}[\n{}{}]$ defined in Eq.~\eqref{eq:exc_def}. -Here, we restrict our study to the case of a two-state ensemble (\ie, $\Nens = 2$) where both the ground state ($I=0$) and the first doubly-excited state ($I=1$) are considered. +Here, we restrict our study to the case of a two-state ensemble (\ie, $\nEns = 2$) where both the ground state ($I=0$) and the first doubly-excited state ($I=1$) are considered. Thus, we have $0 \le \ew{} \le 1/2$. The generalisation to a larger number of states (in particular the inclusion of the first singly-excited state) is trivial and left for future work. @@ -481,7 +489,7 @@ This embedding procedure can be theoretically justified by the generalised adiab \label{eq:GACE} \E{\xc}{\bw}[\n{}{}] = \E{\xc}{}[\n{}{}] - + \sum_{I=1}^{\Nens-1} \int_0^{\ew{I}} \pdv{\E{\xc}{(0,\ldots,0,\xi,\ew{I},\ldots,\ew{\Nens-1})}[\n{}{}]}{\xi} d\xi, + + \sum_{I=1}^{\nEns-1} \int_0^{\ew{I}} \pdv{\E{\xc}{(0,\ldots,0,\xi,\ew{I},\ldots,\ew{\nEns-1})}[\n{}{}]}{\xi} d\xi, \end{equation} (where $\E{\xc}{}[\n{}{}]$ is the usual ground-state xc functional) originally derived by Franck and Fromager. \cite{Franck_2014} Within this in-principle-exact formalism, the (weight-dependent) xc energy of the ensemble is constructed from the (weight-independent) ground-state functional.