Manu: done with II.A

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Emmanuel Fromager 2020-02-24 19:49:14 +01:00
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@ -336,27 +336,62 @@ we finally recover from Eqs.~\eqref{eq:KS_ens_density} and
\right|_{\n{}{} = \n{\opGam{\bw}}{}}. \right|_{\n{}{} = \n{\opGam{\bw}}{}}.
\end{split} \end{split}
\eeq \eeq
Note that, when $\bw=0$, the ensemble correlation functional becomes the Note that, when $\bw=0$, the ensemble correlation functional reduces to the
conventional (ground-state) correlation functional $E_{\rm c}[n]$ and conventional (ground-state) correlation functional $E_{\rm c}[n]$. As a
the ensemble density reduces to the ground-state one $n^{(0)}$. As a result, the regular KS-DFT expression is recovered from
result, Eq.~(\ref{eq:exact_ener_level_dets}) for the ground-state energy:
Eq.~(\ref{eq:exact_ener_level_dets}) can be simplified as follows:
\beq \beq
\E{}{(0)}=\mel*{\Det{(0)}}{\hH}{\Det{(0)}} +
\E{c}{}[\n{\Det{(0)}}{}],
\eeq
or, equivalently,
\beq\label{eq:gs_ener_level_gs_lim}
\begin{split} \begin{split}
\E{}{(I)} \E{}{(0)}&=\mel*{\Det{(0)}}{\hat{H}[\n{\Det{(0)}}{}]}{\Det{(0)}}
& = \mel*{\Det{(I)}}{\hH}{\Det{(I)}} + \E{c}{}[\n{}{(0)}]
\\ \\
& + \int \fdv{\E{c}{}[\n{}{(0)}]}{\n{}{}(\br{})} &
\qty[ \n{\Det{(I)}}{}(\br{}) - \n{\opGam{\bw}}{}(\br{}) ] d\br{} +
\\ \E{c}{}[\n{\Det{(0)}}{}]
&+ -\int \fdv{\E{c}{}[\n{\Det{(0)}}{}]}{\n{}{}(\br{})}\n{\Det{(0)}}{}(\br{})
\sum_{K>0} \qty(\delta_{IK} - \ew{K} ) ,
\left.
\pdv{\E{c}{\bw}[\n{}{}]}{\ew{K}}
\right|_{\n{}{} = \n{\opGam{\bw}}{}}.
\end{split} \end{split}
\eeq \eeq
\titou{Manu, shall we mention that the last term in Eq.~\eqref{eq:exact_ener_level_dets} corresponds to the derivative discontinuity (DD)?} where the ground-state-density-functional Hamiltonian reads
\beq
\hat{H}[\n{\Det{(0)}}{}]=\hH+
\sum^N_{i=1}\fdv{\E{c}{}[\n{\Det{(0)}}{}]}{\n{}{}(\br{i})}.
\eeq
Note that, when divided by the total number $N$ of electrons, the sum of
the last two terms on the right-hand side of
Eq.~(\ref{eq:gs_ener_level_gs_lim}) match the correlation component of
Levy--Zahariev's shift in the KS potential~\cite{Levy_2014}.
The excited-state ($I>0$) energy level expressions
can also be simplified, thus leading to
\beq\label{eq:excited_ener_level_gs_lim}
\begin{split}
\E{}{(I)}
& = \mel*{\Det{(I)}}{\hat{H}[\n{\Det{(0)}}{}]}{\Det{(I)}}
+
\left.
\pdv{\E{c}{\bw}[\n{\Det{(0)}}{}]}{\ew{I}}
\right|_{\bw=0}.
\\
&
+
\E{c}{}[\n{\Det{(0)}}{}]
-\int \fdv{\E{c}{}[\n{\Det{(0)}}{}]}{\n{}{}(\br{})}\n{\Det{(0)}}{}(\br{})
\end{split}
\eeq
As readily seen from Eqs.~(\ref{eq:gs_ener_level_gs_lim}) and
(\ref{eq:excited_ener_level_gs_lim}), any constant shift $\delta
\E{c}{}[\n{\Det{(0)}}{}]/\delta n({\bf r})\rightarrow \delta
\E{c}{}[\n{\Det{(0)}}{}]/\delta n({\bf r})+C$ in the correlation
potential leaves the individual energy levels unchanged. As a result, in
this context,
the correlation derivative discontinuities that is expected to appear when the
excitation process occurs~\cite{Levy_1995} are fully described by the ensemble
correlation derivatives [second term on the right-hand side of
Eq.~(\ref{eq:excited_ener_level_gs_lim})].
%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%
\subsection{One-electron reduced density matrix formulation} \subsection{One-electron reduced density matrix formulation}