Manu: polished the conclusion
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\section{Concluding remarks}
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\label{sec:conclusion}
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In the present article, we have constructed a local, weight-dependent three-state DFA in the context of ensemble DFT.
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The KS-eLDA scheme delivers accurate excitation energies for both single and double excitations, especially within its state-averaged version where the same weights are assigned to each state belonging to the ensemble.
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Generalization to a larger number of states is straightforward and will be investigated in future work.
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We have observed that, although the derivative discontinuity has a non-negligible effect on the excitation energies (especially for the single excitations), its magnitude can be significantly reduced by performing state-averaged calculations instead of zero-weight calculations.
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Using similar ideas, a three-dimensional version \cite{Loos_2009,Loos_2009c,Loos_2010,Loos_2010d,Loos_2017a} of the present eDFA is currently under development to model excited states in molecules and solids.
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Similar to the present excited-state methodology for ensembles, one can easily design a local eDFA for the calculations of the ionization potential, electron affinity, and fundamental gap. \cite{Senjean_2018}
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This can be done by constructing DFAs for the one- and three-electron ground state systems, and combining them with the two-electron DFA in complete analogy with Eqs.~\eqref{eq:ec} and \eqref{eq:ecw}.
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We hope to report on this in the near future.
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A local and ensemble-weight-dependent correlation density-functional approximation
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(eLDA) has been constructed in the context of GOK-DFT for spin-polarized
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triensembles in
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1D. The approach is actually general and can be extended to real
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(three-dimensional)
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systems~\cite{Loos_2009,Loos_2009c,Loos_2010,Loos_2010d,Loos_2017a}
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and larger ensembles in order to
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model excited states in molecules and solids. Work is currently in
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progress in this direction.
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Unlike any standard functional, eLDA incorporates derivative
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discontinuities through its weight dependence. The latter originates
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from the finite uniform electron gas eLDA is
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(partially) based on. The KS-eLDA scheme, where exact exchange is
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combined with eLDA, delivers accurate excitation energies for both
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single and double excitations, especially when an equiensemble is used.
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In the latter case, the same weights are assigned to each state belonging to the ensemble.
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{\it We have observed that, although the derivative discontinuity has a
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non-negligible effect on the excitation energies (especially for the
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single excitations), its magnitude can be significantly reduced by
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performing state-averaged calculations instead of zero-weight
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calculations.}\manu{to be updated ...}
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Let us finally stress that the present methodology can be extended
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straightforwardly to other types of ensembles like, for example, the
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$N$-centered ones, thus allowing for the design an LDA-type functional for the
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calculation of ionization potentials, electron affinities, and
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fundamental gaps. \cite{Senjean_2018,Senjean_2020}.
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Like in the present
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eLDA, such a functional would incorporate the infamous derivative
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discontinuity contribution to the gap through its explicit weight
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dependence. We hope to report on this in the near future.
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\trashEF{This can be done by constructing a functional for the one- and
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three-electron ground-state systems, and combining them with the
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two-electron DFA in complete analogy with Eqs.~\eqref{eq:ec} and
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\eqref{eq:ecw}.}\manu{I find the sentence too technical for a
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conclusion.}
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\section*{Supplementary material}
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