Evaluation of ⟨S2⟩ in density functional theory

The evaluation of < S-2 > in density functional theory (DFT) is considered. Wang [J. Chem. Phys. 102, 3477 (1995)] have derived an approximate, local density expression for < S-2 > and in the present study their formula is evaluated using densities from unrestricted Hartree-Fock (UHF) and a range of DFT exchange-correlation functionals. The results are compared with those obtained by evaluating the conventional UHF expression using the Kohn-Sham orbitals, which is appropriate for the noninteracting system. A generalized gradient approximation for < S-2 > is then proposed and investigated. (c) 2007 American Institute of Physics.