DENSITY-FUNCTIONAL THEORY FOR OPEN-SHELL SINGLET BIRADICALS

The description of open-shell singlet (OSS) sigma-pi biradicals by den sity functional theory (DFT) requires at least a two-configurational ( TC) or, in general, a MC-DFT approach, which bears many unsolved probl ems. These can be avoided by reformulating the TC description in the s pirit of restricted open shell theory for singlets (ROSS) and developi ng an exchange-correlation functional for ROSS-DFT. ROSS-DFT turns out to lead to reliable descriptions of geometry and vibrational frequenc ies for OSS biradicals. The relative energies of the OSS states obtain ed at the ROSS-B3LYP/6-311G(d,p) level are often better than the corre sponding ROSS-MP2 results. However, in those cases where spin polariza tion in a conjugated pi systems plays a role, DFT predicts the triplet state related to the OSS state 2-4 kcal/mol too stable. (C) 1998 Else vier Science B.V. All rights reserved.