Spin‐restricted ensemble‐referenced Kohn–Sham method: basic principles and application to strongly correlated ground and excited states of molecules

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Ensemble density functional theory (DFT) is a novel theoretical approach that is capable of exact treatment of non‐dynamic electron correlation in the ground and excited states of many‐body fermionic systems. In contrast to ordinary DFT, ensemble DFT has not found so far a way to the repertoire of methods of modern computational chemistry, probably owing to the lack of practically affordable implementations of the theory. The spin‐restricted ensemble‐referenced Kohn–Sham (REKS) method represents perhaps the first computational scheme that makes ensemble DFT calculations feasible. The REKS method is based on the rigorous ensemble representation of the energy and the density of a strongly correlated system and provides for an accurate and consistent description of molecular systems the electronic structure of which is dominated by the non‐dynamic correlation. This includes the ground and excited states of molecules undergoing bond breaking/bond formation, the low‐spin states of biradicals and polyradicals, symmetry forbidden chemical reactions and avoided crossings of potential energy surfaces, real intersections between the energy surfaces of the ground and excited states (conical intersections), and many more. The REKS method can be employed in connection with any local, semi‐local and hybrid (global and range‐separated) functional and affords calculations of large and very large molecular systems at a moderate mean‐field cost. WIREs Comput Mol Sci 2015, 5:146–167. doi: 10.1002/wcms.1209This article is categorized under: Structure and Mechanism > Molecular Structures Electronic Structure Theory > Density Functional Theory Theoretical and Physical Chemistry > Spectroscopy