  Density functional theory can be extended to excited states by means of a
unified variational approach for passive state ensembles. This extension
overcomes the restriction of the typical density functional approach to ground
states, and offers useful formal and demonstrated practical benefits. The
correlation energy functional in the generalized case acquires higher
complexity than its ground state counterpart, however. Little is known about
its internal structure nor how to effectively approximate it in general. Here
we show that such a functional can be broken down into natural components,
including what we call "state-" and "density-driven" correlations, with the
former amenable to conventional approximations, and the latter being a unique
feature of ensembles. Such a decomposition, summarised in eq. (6), provides us
with a pathway to general approximations that are able to routinely handle
low-lying excited states. The importance of density-driven correlations is
demonstrated, an approximation for them is introduced and shown to be useful.
