Density functional theory with fractionally occupied frontier orbitals andthe instabilities of the Kohn-Sham solutions for defining diradical transition states: Ring-opening reactions

Density functional theory with fractionally occupied frontier orbitals was combined with an analysis of the instability of the Kohn-Sham solutions and applied to the study of the ring-opening reactions of 1,2-dioxetene, 1,2-d ithiete, 1,2-diselenete, and 1,2-ditellurete. These molecules possess trans ition structures and reactive intermediates for the ring-opening reactions which exhibit strong nondynamical electron correlation. All restricted dens ity functional theory (DFT) solutions for the closed shell transition state structures for all these species are unstable. The solutions with the hybr id Hartree-Fock DFT functionals, B3LYP and B3PW91, are triplet unstable, wh ile for the pure DFT functional BLYP the instability is due to a violation of the Aufbau principle. The same types of instabilities were found for the 1,2-diselenete and 1,2-ditellurete intermediates. Lower energy stable solu tions for the diradical transition structures were found with unrestricted DFT methods allowing fractionally occupied orbitals. This DFT approach indi cates a decrease in the active orbital space from four fractionally occupie d natural orbitals in earlier multireference predictions to two fractionall y occupied Kohn-Sham orbitals. (C) 1999 American Institute of Physics. [S00 21-9606(99)30340-8].