Ess, DH; Johnson, ER; Hu, X; Yang, W, *Singlet-triplet energy gaps for diradicals from fractional-spin density-functional theory.*,
The Journal of Physical Chemistry A, vol. 115 no. 1
(January, 2011),
pp. 76-83 [doi] .
**Abstract:**

*Open-shell singlet diradicals are difficult to model accurately within conventional Kohn-Sham (KS) density-functional theory (DFT). These methods are hampered by spin contamination because the KS determinant wave function is neither a pure spin state nor an eigenfunction of the S(2) operator. Here we present a theoretical foray for using single-reference closed-shell ground states to describe diradicals by fractional-spin DFT (FS-DFT). This approach allows direct, self-consistent calculation of electronic properties using the electron density corresponding to the proper spin eigenfunction. The resulting FS-DFT approach is benchmarked against diradical singlet-triplet gaps for atoms and small molecules. We have also applied FS-DFT to the singlet-triplet gaps of hydrocarbon polyacenes.*