Manu: saving work in the theory section

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Emmanuel Fromager 2020-04-23 12:31:26 +02:00
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@ -19,6 +19,7 @@
\newcommand{\cloclo}[1]{\textcolor{purple}{#1}}
\newcommand{\bruno}[1]{\textcolor{blue}{Bruno: #1}}
\newcommand{\manu}[1]{\textcolor{magenta}{Manu: #1}}
\newcommand{\manuf}[1]{\textcolor{magenta}{#1}}
\newcommand{\trashPFL}[1]{\textcolor{red}{\sout{#1}}}
\newcommand{\trashCM}[1]{\textcolor{purple}{\sout{#1}}}
@ -46,6 +47,7 @@
\newcommand{\hWee}{\Hat{W}_\text{ee}}
\newcommand{\hGam}[1]{\Hat{\Gamma}^{#1}}
\newcommand{\hgam}[1]{\Hat{\gamma}^{#1}}
\newcommand{\hgamdens}[1]{\Hat{\gamma}^{#1}[n]}
\newcommand{\bH}{\boldsymbol{H}}
\newcommand{\hVne}{\Hat{V}_\text{ne}}
\newcommand{\vne}{v_\text{ne}}
@ -120,6 +122,11 @@
\newcommand{\UL}{Instituut-Lorentz, Universiteit Leiden, P.O.~Box 9506, 2300 RA Leiden, The Netherlands}
\newcommand{\VU}{Division of Theoretical Chemistry, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands}
%%% added by Manu %%%
\newcommand{\beq}{\begin{eqnarray}}
\newcommand{\eeq}{\end{eqnarray}}
\begin{document}
\title{Weight Dependence of Local Exchange-Correlation Functionals in Ensemble Density-Functional Theory: Double Excitations in Two-Electron Systems}
@ -213,13 +220,21 @@ semi-empirical as she used experimental excitation energies. It would be
fair to cite her paper. There is also this paper [Journal of Molecular
Structure (Theochem) 571 (2001) 153-161] that I never really understood
but they tried something}.
The present contribution is a small step towards this goal.
The present contribution is a small \manu{too modest I think. ``paves
the way towards ...'' or something like that} step towards this goal.
When one talks about constructing functionals, the local-density approximation (LDA) is never far away.
When one talks about constructing functionals, the local-density
approximation (LDA) is never far away \manu{too ``oral'' style I think}.
The LDA, as we know it, is based on the uniform electron gas (UEG) also known as jellium, an hypothetical infinite substance where an infinite number of electrons ``bathe'' in a (uniform) positively-charged jelly. \cite{Loos_2016}
Although the Hohenberg--Kohn theorems \cite{Hohenberg_1964} are here to provide firm theoretical grounds to DFT, modern KS-DFT rests largely on the presumed similarity between this hypothetical UEG and the electronic behaviour in a real system. \cite{Kohn_1965}
However, Loos and Gill have recently shown that there exists other UEGs which contain finite numbers of electrons (more like in a molecule), \cite{Loos_2011b,Gill_2012} and that they can be exploited to construct LDA functionals. \cite{Loos_2014a,Loos_2014b,Loos_2017a}
Electrons restricted to remain on the surface of a $\cD$-sphere (where $\cD$ is the dimensionality of the surface of the sphere) are an example of finite UEGs (FUEGs). \cite{Loos_2011b}
\manu{It goes much too fast here. One should make a clear distinction
between your previous work with Peter on the ground-state theory. Then
we should refer to our latest work where GOK-DFT is applied
to ringium. In the present work we extend the approach to glomium. As we
did in our previous work we should motivate the use of FUEGs for
developing weight-dependent functionals.}
In particular, we combine these FUEGs with the usual infinite UEG (IUEG) to construct a weigh-dependent LDA correlation functional for ensembles, which is specifically designed to compute double excitations within GOK-DFT, and automatically incorporates the infamous ensemble derivative contribution to the excitation energies through its explicit ensemble weight dependence. \cite{Levy_1995, Perdew_1983}
The paper is organised as follows.
@ -236,24 +251,33 @@ Unless otherwise stated, atomic units are used throughout.
\label{sec:theo}
Let us consider a GOK 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 $\ew{0} \ge \ldots \ge \ew{\nEns-1}$.
\manu{For clarity, I usually exclude $\ew{0}$ from $\bw$ so that $\bw$
only contains the weights that are allowed to vary independently. One
should write explicitly $\ew{0}=1-\sum_{I=1}^{\nEns-1} \ew{I}$ and
define $\bw$ as $\bw = (\ew{1},\ldots,\ew{M-1})$}
The corresponding ensemble energy
\begin{equation}
\E{}{\bw} = \sum_{I=0}^{\nEns-1} \ew{I} \E{}{(I)}
\end{equation}
fulfils the variational principle
fulfils \manu{can be obtained from?} the variational principle
as follows\cite{Gross_1988a}
\begin{eqnarray}\label{eq:ens_energy}
\E{}{\bw} = \min_{\hGam{\bw}} \Tr[\hGam{\bw} \hH],
\end{eqnarray}
where $\hH = \hT + \hWee + \hVne$ contains the kinetic, electron-electron and nuclei-electron interaction potential operators, respectively, $\Tr$ denotes the trace and $\hGam{\bw}$ is a trial density matrix of the form
where $\hH = \hT + \hWee + \hVne$ contains the kinetic,
electron-electron and nuclei-electron interaction potential operators,
respectively, $\Tr$ denotes the trace and $\hGam{\bw}$ is a trial
density matrix operator of the form
\begin{eqnarray}
\hGam{\bw} = \sum_{I=0}^{\nEns - 1} \ew{I} \dyad*{\overline{\Psi}^{(I)}},
\end{eqnarray}
where $\lbrace \overline{\Psi}^{(I)} \rbrace_{0 \le I \le \nEns-1}$ is a set of $\nEns$ orthonormal trial wave functions.
The lower bound of Eq.~\eqref{eq:ens_energy} is reached when the set of wave functions correspond to the exact eigenstates of $\hH$, \ie, $\lbrace \overline{\Psi}^{(I)} \rbrace_{0 \le I \le \nEns-1} = \lbrace \Psi^{(I)} \rbrace_{0 \le I \le \nEns-1}$.
Multiplet degeneracies can be easily handled by assigning the same weight to the degenerate states.
One of the key feature of the GOK ensemble is that individual excitation energies are extracted from the ensemble energy via differentiation with respect to individual weights:
\begin{equation}
Multiplet degeneracies can be easily handled by assigning the same
weight to the degenerate states \cite{Gross_1988b}.
One of the key feature of the GOK ensemble is that individual excitation
energies can be extracted from the ensemble energy via differentiation with respect to individual weights:
\begin{equation}\label{eq:diff_Ew}
\pdv{\E{}{\bw}}{\ew{I}} = \E{}{(I)} - \E{}{(0)} = \Ex{}{(I)}.
\end{equation}
@ -264,39 +288,137 @@ Turning to GOK-DFT, the extension of the Hohenberg--Kohn theorem to ensembles al
\end{equation}
where $\vne(\br{})$ is the external potential and $\F{}{\bw}[\n{}{}]$ is the universal ensemble functional
(the weight-dependent analog of the Hohenberg--Kohn universal functional for ensembles).
In the KS formulation, this functional is decomposed as
In the KS formulation, this functional can be decomposed as
\begin{equation}
\F{}{\bw}[\n{}{}]
= \Ts{\bw}[\n{}{}] + \E{\Hxc}{\bw}[\n{}{}]
= \Tr[ \hgam{\bw} \hT ] + \Tr[ \hgam{\bw} \hWee ],
\end{equation}
\manu{The above equation is wrong (the correlation is missing) and the
notations are ambiguous. I should also say that Tim does not like the
original separation into H and xc. I propose the following reformulation
to get everyone satisfied. I also reorganized the theory for clarity.
\begin{equation}\label{eq:FGOK_decomp}
\F{}{\bw}[\n{}{}]
= \Tr[ \hgamdens{\bw} \hT ]+ \E{\Ha}{}[\n{}{}]+\E{\xc}{\bw}[\n{}{}],
\end{equation}
}
where
$\Ts{\bw}[\n{}{}]$ is the noninteracting ensemble kinetic energy functional,
\manuf{$\Tr[ \hgamdens{\bw} \hT ]=\Ts{\bw}[\n{}{}]$} is the noninteracting ensemble kinetic energy functional,
\begin{equation}
\hgam{\bw} = \sum_{I=0}^{\nEns-1} \ew{I} \dyad{\Det{I}{\bw}}
\hgam{\bw}[n] = \sum_{I=0}^{\nEns-1} \ew{I} \dyad{\Det{I}{\bw}[n]}
\end{equation}
is the density matrix operator, $\lbrace \Det{I}{\bw} \rbrace_{0 \le I \le \nEns-1}$ are single-determinant wave functions (or configuration state functions) built with KS orbitals $\lbrace \MO{p}{\bw}(\br{}) \rbrace_{1 \le p \le \nOrb}$, and
is the \manuf{KS density-functional} density matrix operator, and $\lbrace
\Det{I}{\bw}[n] \rbrace_{0 \le I \le \nEns-1}$ are single-determinant
wave functions (or configuration state functions). \manuf{Their
dependence on the density
is determined from the ensemble density
constraint
\begin{equation}
\label{eq:exc_def}
\begin{split}
\E{\Hxc}{\bw}[\n{}{}]
& = \E{\Ha}{}[\n{}{}] + \E{\xc}{\bw}[\n{}{}]
\\
& = \frac{1}{2} \iint \frac{\n{}{}(\br{}) \n{}{}(\br{}')}{\abs{\br{}-\br{}'}} d\br{} d\br{}'
+ \int \e{\xc}{\bw}(\n{}{}(\br{})) \n{}{}(\br{}) d\br{}
\end{split}
\sum_{I=0}^{\nEns-1} \ew{I} n_{\Det{I}{\bw}[n]}(\br)=n(\br).
\end{equation}
is the ensemble Hartree-exchange-correlation (Hxc) functional.
Note that the weight-independent Hartree functional $\E{\Ha}{}[\n{}{}]$ causes the infamous ghost-interaction error \cite{Gidopoulos_2002, Pastorczak_2014, Alam_2016, Alam_2017, Gould_2017} in eDFT, which is supposed to be cancelled by the weight-dependent xc functional $\E{\xc}{\bw}[\n{}{}]$.
Note that the original decomposition \cite{Gross_1988b} shown in Eq.~(\ref{eq:FGOK_decomp}), where the
conventional (weight-independent) Hartree functional
\beq
\E{\Ha}{}[\n{}{}]=\frac{1}{2} \iint \frac{\n{}{}(\br{}) \n{}{}(\br{}')}{\abs{\br{}-\br{}'}} d\br{} d\br{}'
\eeq
is separated
from the (weight-dependent) exchange-correlation (xc) functional, is
formally exact. In practice, the use of such a decomposition might be
problematic as inserting an ensemble density into $\E{\Ha}{}[\n{}{}]$
causes the infamous ghost-interaction error \cite{Gidopoulos_2002,
Pastorczak_2014, Alam_2016, Alam_2017, Gould_2017}. The latter should in
principle be removed by the exchange component of the ensemble xc
functional $\E{\xc}{\bw}[\n{}{}]\equiv
\E{\ex}{\bw}[\n{}{}]+\E{\co}{\bw}[\n{}{}]$, as readily seen from the
exact expression
\beq
\E{\ex}{\bw}[\n{}{}]=\sum_{I=0}^{\nEns-1} \ew{I}\bra{\Det{I}{\bw}[n]}\hat{W}_{\rm ee}\ket{\Det{I}{\bw}[n]}
-\E{\Ha}{}[\n{}{}].
\eeq
The minimum in Eq.~(\ref{eq:Ew-GOK}) is reached when the density $n$
equals the exact ensemble one
\beq\label{eq:nw}
n^{\bw}(\br)=\sum_{I=0}^{\nEns-1}
\ew{I}n_{\Psi_I}(\br).
\eeq
The minimizing density-functional KS
wavefunctions $\left\{\Det{I}{\bw}\left[n^{\bw}\right]\right\}_{0\leq
I\leq M-1}$ are constructed from (weight-dependent) KS orbitals
$\lbrace \MO{p}{\bw}(\br{}) \rbrace_{1 \le p \le
\nOrb}$. The latters are determined by solving the ensemble KS equation
\begin{equation}
\label{eq:eKS}
\qty{ \hHc(\br{}) + \fdv{\E{\Hxc}{\bw}[\n{}{\bw}]}{\n{}{}(\br{})}} \MO{p}{\bw}(\br{}) = \eps{p}{\bw} \MO{p}{\bw}(\br{}),
\end{equation}
where $\hHc(\br{}) = -\nabla^2/2 + \vne(\br{})$, and
\begin{equation}
\fdv{\E{\Hxc}{\bw}[\n{}{}]}{\n{}{}(\br{})}
=
\int \frac{\n{}{}(\br{}')}{\abs{\br{}-\br{}'}} d\br{}'
%\fdv{\E{\Ha}{}[\n{}{}]}{\n{}{}(\br{})}
+ \fdv{\E{\xc}{\bw}[\n{}{}]}{\n{}{}(\br{})}.
\end{equation}
The ensemble density can be obtained directly (and exactly, if no
approximation is made) from those orbitals:
\beq
\n{}{\bw}(\br{})=\sum_{I=0}^{\nEns-1} \ew{I}\left(\sum_{p}^{\nOrb}
\ON{p}{(I)} [\MO{p}{\bw}(\br{})]^2\right),
\eeq
where $\ON{p}{(I)}$ denotes the occupation of $\MO{p}{\bw}(\br{})$ in
the $I$th KS wave function $\Det{I}{\bw}\left[n^{\bw}\right]$. Turning
to the excitation energies, they can be extracted from the
density-functional ensemble as follows [see Eqs. ({\ref{eq:diff_Ew}})
and ({\ref{eq:Ew-GOK}}) and Refs.
\cite{Gross_1988b,Deur_2019}]:
\beq
\label{eq:dEdw}
\Omega^{(I)}= \Eps{I}{\bw} - \Eps{0}{\bw} + \left. \pdv{\E{\xc}{\bw}[\n{}{}]}{\ew{I}} \right|_{\n{}{} = \n{}{\bw}},
\eeq
where
\begin{equation}
\label{eq:KS-energy}
\Eps{I}{\bw} = \sum_{p}^{\nOrb} \ON{p}{(I)} \eps{p}{\bw}
\end{equation}
is the energy of the $I$th KS state.
}%%%%%% end manuf
From the GOK-DFT ensemble energy expression in Eq.~\eqref{eq:Ew-GOK}, we obtain \cite{Gross_1988b,Deur_2019}
Equation \eqref{eq:dEdw} is our working equation for computing excitation energies from a practical point of view.
Note that the individual KS densities
$\n{\Det{I}{\bw}\left[n^{\bw}\right]}{}(\br{})=\sum_{p}^{\nOrb}
\ON{p}{(I)} [\MO{p}{\bw}(\br{})]^2$ do
not necessarily match the \textit{exact} (interacting) individual-state densities as the non-interacting KS ensemble is expected to reproduce the true interacting ensemble density $\n{}{\bw}(\br{})$ defined in Eq.~\eqref{eq:nw}, and not each individual density.
Nevertheless, these densities can still be extracted in principle exactly from the KS ensemble as shown by Fromager. \cite{Fromager_2020}
\manuf{
In the following, we will work at the (weight-dependent) LDA
level of approximation, \ie
\beq
\E{\xc}{\bw}[\n{}{}]
&\overset{\rm LDA}{\approx}&
\int \e{\xc}{\bw}(\n{}{}(\br{})) \n{}{}(\br{}) d\br{}
\\
\fdv{\E{\xc}{\bw}[\n{}{}]}{\n{}{}(\br{})}
&\overset{\rm LDA}{\approx}&
\left. \pdv{\e{\xc}{\bw{}}(\n{}{})}{\n{}{}} \right|_{\n{}{} = \n{}{}(\br{})} \n{}{}(\br{}) + \e{\xc}{\bw{}}(\n{}{}(\br{})).
\eeq
In the following, we adopt the usual decomposition, and write down the weight-dependent xc functional as
\begin{equation}
\e{\xc}{\ew{}}(\n{}{}) = \e{\ex}{\ew{}}(\n{}{}) + \e{\co}{\ew{}}(\n{}{}),
\end{equation}
where $\e{\ex}{\ew{}}(\n{}{})$ and $\e{\co}{\ew{}}(\n{}{})$ are the weight-dependent exchange and correlation functionals, respectively.
}
%%%%%%%%%%%%%%%%
%%%%%%% Manu: stuff that I removed from the first version %%%%%
\iffalse%%%%
\begin{equation}
\begin{split}
\label{eq:dEdw}
\pdv{\E{}{\bw}}{\ew{I}}
& = \E{}{(I)} - \E{}{(0)}
\\
& = \Eps{I}{\bw} - \Eps{0}{\bw} + \left. \pdv{\E{\xc}{\bw}[\n{}{}]}{\ew{I}} \right|_{\n{}{} = \n{}{\bw}(\br{})},
& = \Eps{I}{\bw} - \Eps{0}{\bw} + \left. \pdv{\E{\xc}{\bw}[\n{}{}]}{\ew{I}} \right|_{\n{}{} = \n{}{\bw}},
\end{split}
\end{equation}
where
@ -308,16 +430,30 @@ where
\n{\Det{I}{\bw}}{}(\br{}) & = \sum_{p}^{\nOrb} \ON{p}{(I)} [\MO{p}{\bw}(\br{})]^2
\end{align}
are the ensemble and individual one-electron densities, respectively,
and
\begin{equation}
\label{eq:KS-energy}
\Eps{I}{\bw} = \sum_{p}^{\nOrb} \ON{p}{(I)} \eps{p}{\bw}
\label{eq:exc_def}
\begin{split}
\E{\Hxc}{\bw}[\n{}{}]
& = \E{\Ha}{}[\n{}{}] + \E{\xc}{\bw}[\n{}{}]
\\
& = \frac{1}{2} \iint \frac{\n{}{}(\br{}) \n{}{}(\br{}')}{\abs{\br{}-\br{}'}} d\br{} d\br{}'
+ \int \e{\xc}{\bw}(\n{}{}(\br{})) \n{}{}(\br{}) d\br{}
\end{split}
\end{equation}
is the ensemble Hartree-exchange-correlation (Hxc) functional.
Note that the weight-independent Hartree functional $\E{\Ha}{}[\n{}{}]$
is the weight-dependent KS energy of state $I$, and $\eps{p}{\bw}$ is the KS orbital energy associated with $\MO{p}{\bw}(\br{})$ [$\ON{p}{(I)}$ being its occupancy for the state $I$].
The latters are determined by solving the ensemble KS equation
\begin{equation}
\label{eq:eKS}
\qty{ \hHc(\br{}) + \fdv{\E{\Hxc}{\bw}[\n{}{\bw}]}{\n{}{}(\br{})}} \MO{p}{\bw}(\br{}) = \eps{p}{\bw} \MO{p}{\bw}(\br{}),
\end{equation}
built with KS orbitals $\lbrace \MO{p}{\bw}(\br{}) \rbrace_{1 \le p \le
\nOrb}$,
where $\hHc(\br{}) = -\nabla^2/2 + \vne(\br{})$, and
\begin{equation}
\fdv{\E{\Hxc}{\bw}[\n{}{}]}{\n{}{}(\br{})}
@ -333,16 +469,8 @@ is the Hxc potential, with
& = \left. \pdv{\e{\xc}{\bw{}}(\n{}{})}{\n{}{}} \right|_{\n{}{} = \n{}{}(\br{})} \n{}{}(\br{}) + \e{\xc}{\bw{}}(\n{}{}(\br{})).
\end{align}
\end{subequations}
Equation \eqref{eq:dEdw} is our working equation for computing excitation energies from a practical point of view.
Note that the individual densities $\n{\Det{I}{\bw}}{}(\br{})$ defined in Eq.~\eqref{eq:nI} do not match the \textit{exact} individual-state densities as the non-interacting KS ensemble is expected to reproduce the true interacting ensemble density $\n{}{\bw}(\br{})$ defined in Eq.~\eqref{eq:nw}, and not each individual density.
Nevertheless, these densities can still be extracted in principle exactly from the KS ensemble as shown by Fromager. \cite{Fromager_2020}
In the following, we adopt the usual decomposition, and write down the weight-dependent xc functional as
\begin{equation}
\e{\xc}{\ew{}}(\n{}{}) = \e{\ex}{\ew{}}(\n{}{}) + \e{\co}{\ew{}}(\n{}{}),
\end{equation}
where $\e{\ex}{\ew{}}(\n{}{})$ and $\e{\co}{\ew{}}(\n{}{})$ are the weight-dependent exchange and correlation functionals, respectively.
%%%%% end stuff removed by Manu %%%%%%
\fi%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%% COMPUTATIONAL DETAILS %%%