Manu: done with my revisions
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@ -663,9 +663,9 @@ expressions. It is in fact difficult to readily distinguish from
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Eqs.~(\ref{eq:Xic}) and (\ref{eq:Upsic}) purely (uncoupled) individual
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contributions from mixed ones. For that purpose, we may
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consider a density regime which has a weak deviation from the uniform
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one. In such a regime, for which eLDA is a reasonable approximation, the
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one. In such a regime, where eLDA is a reasonable approximation, the
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deviation of the individual densities from the ensemble one will be
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weak. As a result,
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small. As a result,
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we can} Taylor expand the density-functional
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correlation contributions
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around the $I$th KS state density
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@ -682,10 +682,11 @@ Therefore, it can be identified as
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an individual-density-functional correlation energy where the density-functional
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correlation energy per particle is approximated by the ensemble one for
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all the states within the ensemble. \manurev{This perturbation expansion
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may not hold in realistic systems, which may deviate significantly from
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the uniform density regime. Nevertheless, it
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gives more insight into the eLDA approximation and becomes useful when
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rationalizing its performance, as illustrated in Sec. \ref{sec:res}.\\}
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is of course less relevant for the (more realistic) systems that exhibit significant
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deviations from the uniform
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density regime. Nevertheless, it
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gives more insight into the eLDA approximation and it becomes useful when
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it comes to rationalize its performance, as illustrated in Sec. \ref{sec:res}.\\}
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Let us stress that, to the best of our knowledge, eLDA is the first
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density-functional approximation that incorporates ensemble weight
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dependencies explicitly, thus allowing for the description of derivative
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@ -987,7 +988,9 @@ Its Tamm-Dancoff approximation version (TDA-TDLDA) is also considered. \cite{Dr
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Concerning the ensemble calculations, two sets of weight are tested: the zero-weight
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(ground-state) limit where $\bw = (0,0)$ and the
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equi-triensemble (or equal-weight state-averaged) limit where $\bw = (1/3,1/3)$.
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\titou{Note that a zero-weight calculation does correspond to a conventional ground-state KS calculation with exact exchange and LDA correlation.}
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\titou{Note that a zero-weight calculation does correspond to a
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\trashEF{conventional} ground-state KS calculation with \manu{$100\%$} exact exchange and LDA correlation.}
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\manu{Manu: OK?}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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@ -1014,6 +1017,12 @@ linearly-interpolated ensemble energy) is represented
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in Fig.~\ref{fig:EvsW} as a function of $\ew{1}$ or $\ew{2}$ while
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fulfilling the restrictions on the ensemble weights to ensure the GOK
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variational principle [\ie, $0 \le \ew{2} \le 1/3$ and $\ew{2} \le \ew{1} \le (1-\ew{2})/2$].
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\manurev{More precisely, we follow a continuous path that connects
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ground-state [$\bw=(0,0)$] and equiensemble [$\bw=(1/3,1/3)$]
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calculations. For convenience, we use two connected paths. The first
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one, for which $\ew{2}=0$ and $0\leq \ew{1}\leq 1/3$, relies on the
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biensemble while the second one is defined as follows:
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$\ew{1}=1/3$ and $0\leq \ew{2}\leq 1/3$.}
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To illustrate the magnitude of the ghost-interaction error, we report the KS-eLDA ensemble energy with and without GIC as explained above {[see Eqs.~\eqref{eq:Ew-GIC-eLDA} and \eqref{eq:Ew-eLDA}]}.
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As one can see in Fig.~\ref{fig:EvsW}, without GIC, the
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ensemble energy becomes less and less linear as $L$
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@ -1082,7 +1091,7 @@ systematically enhances the weight dependence, due to the lowering of the
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ground-state energy, as $\ew{2}$ increases.
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The reverse is observed for the second excited state.
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\manurev{Finally, we notice that the crossover point of the
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first excited-state energies based on
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first-excited-state energies based on
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bi- and triensemble calculations, respectively, disappears in the strong correlation
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regime [see the right panel of Fig. \ref{fig:EIvsW}], thus illustrating
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the importance of (individual and ensemble) densities, in
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@ -1123,7 +1132,7 @@ Overall, one clearly sees that, with
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equal weights, KS-eLDA yields accurate excitation energies for both single and double excitations.
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This conclusion is verified for smaller and larger numbers of electrons
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(see {\SI}).
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\titou{Except for the two-electron system where we observe cases of underestimation, eLDA usually overestimates double excitations, as evidenced by the numerical data gathered in the {\SI}).}
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\titou{Except for the two-electron system where we observe cases of underestimation, eLDA usually overestimates double excitations, as evidenced by the numerical data gathered in the {\SI}.}
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%%% FIG 4 %%%
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\begin{figure*}
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