  We report a local, weight-dependent correlation density-functional
approximation that incorporates information about both ground and excited
states in the context of density-functional theory for ensembles (eDFT). This
density-functional approximation for ensembles is specially designed for the
computation of single and double excitations within Gross--Oliveira--Kohn (GOK)
DFT (i.e., eDFT for neutral excitations), and can be seen as a natural
extension of the ubiquitous local-density approximation in the context of
ensembles. The resulting density-functional approximation, based on both finite
and infinite uniform electron gas models, automatically incorporates the
infamous derivative discontinuity contributions to the excitation energies
through its explicit ensemble weight dependence. Its accuracy is illustrated by
computing single and double excitations in one-dimensional many-electron
systems in the weak, intermediate and strong correlation regimes. Although the
present weight-dependent functional has been specifically designed for
one-dimensional systems, the methodology proposed here is general, i.e.,
directly applicable to the construction of weight-dependent functionals for
realistic three-dimensional systems, such as molecules and solids.
