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.
Highlights
Biradicals such as p-didehydrobenzene [1] or α,3-didehydrotoluene [2, 3] have attracted considerable attention in the last ten years because they are the intermediates of the Bergman reaction [4] of enediynes and determine the biological activity of the latter compounds, in particular their potential as antitumor drugs
density functional theory (DFT) has become an attractive tool for the investigation of enediyne biradicals because DFT and in particular unrestricted DFT (UDFT) lead to reasonable descriptions of biradicals [1, 11,12,13, 17,18,19,20,21,22,23] and in this way provides a convenient way of circumventing the use of much more expensive ab initio methods in these cases
Reliable descriptions can only be obtained if a multireference approach such as MRCI or MRCC [28] is used or alternatively a single-determinant method covering a high amount of dynamic electron correlation is applied
Summary
Biradicals such as p-didehydrobenzene [1] or α,3-didehydrotoluene [2, 3] have attracted considerable attention in the last ten years because they are the intermediates of the Bergman reaction [4] of enediynes and determine the biological activity of the latter compounds, in particular their potential as antitumor drugs. Reliable descriptions can only be obtained if a multireference approach such as MRCI or MRCC [28] is used or alternatively a single-determinant method covering a high amount of dynamic electron correlation is applied Methods such as CCSD(T) or BD(T) will partially compensate for the deficiencies of a single-determinant starting wave function and in this way provide a reasonable account of the properties of biradicals [10, 12, 16, 18, 29]. We will critically discuss the question to which extent DFT and in particular UDFT can be used for a reliable description of biradicals with multireference character For this purpose, we will consider simple biradicals such as methylene in its OSS biradical state (1B1, 1), α, 3-didehydrotoluene (1A , 2) or 1, 4-didehydrobenzene (1Ag, 3, Scheme 1).
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