Diradical character from the local spin analysis.

Diradical species are analyzed in light of the local spin analysis. The atomic and diatomic contributions to the overall 〈Ŝ(2)〉 value are used to detect the diradical character of a number of molecular species mostly in their singlet state, for which no spin density exists. A general procedure for the quantification of diradical character for both singlet and triplet states is achieved by using a recently introduced index that measures the deviation of an actual molecule from an ideal system of perfectly localized spin centers. The index is of general applicability and can be easily determined in equal footing from a multireference or an open-shell single-determinant wave function.