Can Unrestricted Density-Functional Theory Describe Open Shell Singlet Biradicals?

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.

Pyrazolium- and 1,2-Cyclopentadiene-Based Ligands as σ-Donors: a Theoretical Study of Electronic Structure and Bonding

“…However, unrestricted DFT is known to fail by construction in describing pure singlet diradicals for which a Kohn-Sham determinant does not represent a true electronic state. 27 In the current case, this leads to artificial stabilization of the 1 A2 state at the DFT level, giving a false global minimum and a failure in finding a diradical component within the chiral molecular framework. This conclusion is further supported by subsequent calculations at the CASPT2 level.…”

“…10b In general, DFT in its standard formalism is not the preferred approach for studies of molecules in open-shell singlet states due to the limited applicability of unrestricted approaches in accurate description of near-degeneracy effects. 27 For parent 1,2-cyclopentadiene, this means that DFT can be reliably employed only for the C2v The PES calculated for 1,2-cyclopentadiene is reminiscent of that reported for 1,2,4,6-cycloheptatetraene, 6. 28 Although the energy landscapes of the two molecules are qualitatively very similar (Figure 2), there are some quantitative differences as well.…”

Multideterminant assessment of mean-field methods for the description of electron transfer in the weak-coupling regime

“…Their performance in describing open-shell molecular systems, such as singlet biradicals or dissociating molecules, classified as type-I and type-II systems, respectively, have also been explored 20,54,55 .…”

Spin Symmetry Requirements in Density Functional Theory: The Proper Way to Predict Magnetic Coupling Constants in Molecules and Solids

“…The use of the unprojected results may obscure some important issues [4,5,28,[56][57][58][75][76][77] and lead to some contradictions [5,28]. Still, the auxiliary nature of the KS orbitals makes this procedure at least controversial [4,5,78]. Electronic structure calculations for solid state systems face an additional difficulty.…”