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 〈Ŝ2〉 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 functionFinancial help has been furnished by the Spanish MICINN Projects No. CTQ2011-23441/BQU. Financial support from MICINN and the FEDER fund (European Fund for Regional Development) was also provided by grant UNGI08-4E-003. Financial support from the Generalitat de Catalunya (SGR528 and Xarxa de Referencia en Quimica Teorica i Computacional) is also acknowledged. E.R-C. acknowledges support from the Spanish FPU program (Grant No. AP2008-01231) and from the EU under a Marie Curie Career Integration grant (PCI09-GA-2011-294240