title: Singlet-triplet energy gaps for diradicals from fractional-spin density-functional theory.
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.
authors.affiliation: Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, USA. dhe@chem.byu.edu
authors.forename: Daniel H
authors.initials: DH
authors.lastname: Ess
authors.forename: Erin R
authors.initials: ER
authors.lastname: Johnson
authors.forename: Xiangqian
authors.initials: X
authors.lastname: Hu
authors.forename: Weitao
authors.initials: W
authors.lastname: Yang