A comparative study of the singlet–triplet energy separation of carbenes using density functional theory and coupled-cluster methods

The singlet–triplet energy separation (ΔES–T) of carbenes has been studied using density functional theory (DFT) and the single, double, plus perturbative triple excitations coupled-cluster {CCSD(T)} approaches. ΔES–T predictions from DFT were compared with those of CCSD(T). We found that BLYP and G96LYP predictions are closest to the CCSD(T) results, with the average absolute deviations within 1 kcal/mol. Many of the tested functionals overestimate ΔES–T. It was observed that best comparisons are displayed by the choice of correlation (LYP) functionals, while the effect of choosing among exchange functionals is less significant. The hybrid functionals provide poorer, overestimated predictions for ΔES–T.