Singlet-triplet gaps in diradicals by the spin-flip approach: A benchmark study
The spin-flip approach has been applied to calculate vertical and adiabatic energy separations between low-lying singlet and triplet states in diradicals. The spin-flip model describes both closed and open shell singlet and (low-spin) triplet states within a single reference formalism as spin-flipping, e. g., α→β, excitations from a high-spin triplet (Ms = 1) reference state. Since both dynamical and non-dynamical correlation effects are much smaller for the high-spin triplet states than for the corresponding singlet states, the spin-flip models yield systematically more accurate results than their traditional (non spin-flip) counterparts. For all the diradicals studied in this work, the spin-flip variant of the coupled-cluster model with double excitations yields energy separations which are within less than 3 kcal/mol of the experimental or the highly accurate multi-reference values. In most cases the errors are about 1 kcal/mol.