Use of noncollinear exchange-correlation potentials in multiplet resolutions by time-dependent density functional theory

The resolution of spin-multiplets in an electronic system with a spin-degenerate ground state requires a method that can deal with spin-flip excitations. We recently showed that time-dependent density functional theory (TDDFT) can treat such excitations provided that use is made of a noncollinear exchange-correlation (XC) potential. The new formulation is a generalization of conventional TDDFT. The form of the new formulation is exactly the same as that of the conventional TDDFT for transitions that do not involve spin-flips. However, the noncollinear TDDFT formulation allows in addition for spin-flip transitions. In the present account, we explain in more detail why a noncollinear formulation of the XC potential is required. (C) 2006 Wiley Periodicals, Inc.