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.