A density functional method for degenerate spin-multiplet components

The Kohn–Sham spin-density functional method is adapted to the treatment of spin multiplet degeneracy by replacing the variables ρα and ρβ with transforms of the charge density and the density of effectively unpaired electrons. In essence, the modification ensures the degeneracy of Kohn–Sham determinants that differ only by the orientation of spins of unpaired electrons. As demonstrated with the N2 and O2 molecules, the approach solves the size-consistency dilemma for a manifold of molecular states associated with the same separated atom limit.