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

Abstract

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., α→β, excitations from a high-spin (Ms=1) 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., closed- and open-shell singlets, as well as low-spin triplets) by linear response from a reference high-spin triplet (Ms=1) 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.