Singularities in the behavior of density functionals in predictions of singlet biradicals: The 1,2-dichalcogenins

The performance of conventional exchange-correlation functionals for the de scription of singlet biradicals which possess a small admixture of an auxil iary electronic state was examined using a broken symmetry spin unrestricte d density functional approach. A pure density functional, BLYP, and the hyb rid B3LYP and BHandHLYP exchange-correlation functionals were employed to s tudy the thermal ring opening of 1,2-dichalcogenins and the subsequent form ation of the bicyclic products. A stepwise mechanism was predicted for the reaction of the 1,2-dithiin and 2-selenathiin rings and no biradical struct ures were located. For 1,2-diselenin, the biradical stepwise mechanism comp etes energetically with a concerted reaction which is slightly favored. For 1,2-ditellurin, a high-energy open intermediate with considerable biradica l character was located. The hybrid functionals show an increase in biradic al character with an increase in the percentage of Hartree-Fock exchange in corporated into the exchange functional. The instability of the restricted Kohn-Sham solutions was used to indicate the existence of biradical structu res with lower energy. Interesting incorrect behavior of certain of the hyb rid functionals was noted. The instability of the restricted BHandHLYP solu tion results from too large a contribution of Hartree-Fock exchange. An une xpected stability of the restricted B3LYP solution was found and attributed to cancellation of two different sources of instability: one from the pure density functional and the other from the inclusion of Hartree-Fock exchan ge. (C) 2000 American Institute of Physics. [S0021-9606(00)30223-9].