  Over the years, several schemes have been proposed to describe multireference
systems with Kohn-Sham Density Functional Theory. Problematic is the
combination of two aspects: the Kohn-Sham reference wavefunction is usually
taken to be a single Slater determinant, and approximate exchange-correlation
functionals are typically derived form the local density approximation. In this
work, we develop a theoretical framework that foregoes the single Slater
determinant and instead employs thermal states as reference states for
zero-temperature interacting systems. We provide convenient definitions of
static and dynamic correlation functionals via an adiabatic connection
approach. The formalism and computational results indicate that the entropic
term of the thermal reference state is a good approximation to the static
correlation functional. Hence, this work validates several reported results in
the literature and motivates additional developments of static correlation
density functionals.
