The spin–flip approach within time-dependent density functional theory: Theory and applications to diradicals

An extension of density functional theory to situations with significant nondynamical correlation is presented. The method is based on the spin–flip ~SF! approach which is capable of describing multireference wave functions within a single reference formalism as spin–flipping, e.g., a→b, excitations from a high-spin (M s51) triplet reference state. An implementation of the spin–flip approach within the Tamm–Dancoff approximation to time-dependent density functional theory ~TDDFT! is presented. The new method, SF-TDDFT/TDA or simply SF-DFT, describes target states ~i.e., closedand open-shell singlets, as well as low-spin triplets! by linear response from a reference high-spin triplet (M s51) Kohn–Sham state. Contrary to traditional TDDFT, the SF-DFT response equations are solved in a subspace of spin–flipping operators. The method is applied to bond-breaking ~ethylene torsional potential!, and equilibrium properties of eight diradicals. The results demonstrate significant improvement over traditional Kohn–Sham DFT, particularly for 50/50 hybrid functional. © 2003 American Institute of Physics. @DOI: 10.1063/1.1545679#