Spin-flip time dependent density functional theory for singlet-triplet splittings in σ,σ-biradicals

A spin-flip time dependent density functional theory approach with hybrid non-collinear exchange-correlation kernels has been applied to investigate the energy gap between the lowest singlet and triplet states of σ,σ-biradicals. The obtained results indicate that spin-flip time dependent density functional theory is capable to predict the correct ordering of singlet and triplet states among all investigated biradicals and that it gives estimates of singlet-triplet splittings in good agreement with high level correlated ab initio calculations. The theory provides a superior accuracy compared to the conventional broken-symmetry unrestricted density functional theory methods.