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

摘要

The performance of conventional exchange-correlation functionals for the description of singlet biradicals which possess a small admixture of an auxiliary electronic state was examined using a broken symmetry spin unrestricted density functional approach. A pure density functional, BLYP, and the hybrid B3LYP and BHandHLYP exchange-correlation functionals were employed to study the thermal ring opening of 1,2-dichalcogenins and the subsequent formation of the bicyclic products. A stepwise mechanism was predicted for the reaction of the 1,2-dithiin and 2-selenathiin rings and no biradical structures were located. For 1,2-diselenin, the biradical stepwise mechanism competes energetically with a concerted reaction which is slightly favored. For 1,2-ditellurin, a high-energy open intermediate with considerable biradical character was located. The hybrid functionals show an increase in biradical character with an increase in the percentage of Hartree-Fock exchange incorporated into the exchange functional. The instability of the restricted Kohn-Sham solutions was used to indicate the existence of biradical structures with lower energy. Interesting incorrect behavior of certain of the hybrid functionals was noted. The instability of the restricted BHandHLYP solution results from too large a contribution of Hartree-Fock exchange. An unexpected 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 exchange. (C) 2000 American Institute of Physics. [S0021-9606(00)30223-9]. [References: 42]