  For electronic systems with multi-reference (MR) character, Kohn-Sham density
functional theory (KS-DFT) with the conventional exchange-correlation (xc)
energy functionals can lead to incorrect spin densities and related properties.
For example, for H2 dissociation, the spin-restricted and spin-unrestricted
solutions obtained with the same xc energy functional in KS-DFT can be
distinctly different, yielding the unphysical spin-symmetry breaking effects in
the spin-unrestricted solutions. Recently, thermally-assisted-occupation
density functional theory (TAO-DFT) has been shown to resolve the
aforementioned spin-symmetry breaking, when the fictitious temperature is
properly chosen. In this work, a response theory based on TAO-DFT is developed
to demonstrate that TAO-DFT with a sufficiently large fictitious temperature
can always resolve the unphysical spin-symmetry breaking in MR systems. To
further support this, TAO-DFT calculations with various fictitious temperatures
are performed for the dissociation of H2, N2, He2, and Ne2 as well as the
twisted ethylene.
