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Can Unrestricted Density-Functional Theory Describe Open Shell Singlet Biradicals?
Department of Theoretical Chemistry, G¨oteborg University, Reutersgatan 2, S-413 20 G¨oteborg, Sweden
* Author to whom correspondence should be addressed.
Received: 29 August 2001; Accepted: 17 December 2001 / Published: 25 April 2002
Abstract: Unrestricted density functional theory (UDFT) can be used for the description of open-shell singlet (OSS) biradicals provided a number of precautions are considered. Biradicals that require a two-determinantal wave function (e.g. OSS state of carbenes) cannot be described by UDFT for principal reasons. However, if the overlap between the open-shell orbitals is small (the single electrons are located at different atomic centers) errors become small and, then, the principal failure of UDFT in these cases is not apparent and may even be disguised by the fact that UDFT has the advantage of describing spin polarization better than any restricted open shell DFT method. In the case of OSS biradicals with two- or multiconfigurational character (but a onedeterminantal form of the leading configuration), reasonable results can be obtained by broken-symmetry (BS)-UDFT, however in each case this has to be checked. In no case is it reasonable to lower the symmetry of a molecule to get a suitable UDFT description. Hybrid functionals such as B3LYP perform better than pure DFT functionals in BS-UDFT calculations because the former reduce the self-interaction error of DFT exchange functionals, which mimics unspecified static electron correlation effects, so that the inclusion of specific static electron correlation effects via the form of the wavefunction becomes more effective.
Keywords: Unrestricted Density Functional Theory (UDFT); non-dynamic electron correlation; biradicals; spin contamination
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Gräfenstein, J.; Kraka, E.; Filatov, M.; Cremer, D. Can Unrestricted Density-Functional Theory Describe Open Shell Singlet Biradicals? Int. J. Mol. Sci. 2002, 3, 360-394.
Gräfenstein J, Kraka E, Filatov M, Cremer D. Can Unrestricted Density-Functional Theory Describe Open Shell Singlet Biradicals? International Journal of Molecular Sciences. 2002; 3(4):360-394.
Gräfenstein, Jürgen; Kraka, Elfi; Filatov, Michael; Cremer, Dieter. 2002. "Can Unrestricted Density-Functional Theory Describe Open Shell Singlet Biradicals?" Int. J. Mol. Sci. 3, no. 4: 360-394.