Kohn–Sham density functional theory has been tremendously successful in chemistry and physics. Yet, it is unable to describe the energy degeneracy of spin-multiplet components with any approximate functional. This work features two contributions. (1) We present a multistate density functional theory (MSDFT) to represent spin-multiplet components and to determine multiplet energies. MSDFT is a hybrid approach, taking advantage of both wave function theory and density functional theory. Thus, the wave functions, electron densities and energy density-functionals for ground and excited states and for different components are treated on the same footing. The method is illustrated on valence excitations of atoms and molecules. (2) Importantly, a key result is that for cases in which the high-spin components can be determined separately by Kohn–Sham density functional theory, the transition density functional in MSDFT (which describes electronic coupling) can be defined rigorously. The numerical results may be explored to design and optimize transition density functionals for configuration coupling in multiconfigurational DFT.

中文翻译：

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自旋多重成分和能量分裂的多态密度泛函理论

Kohn-Sham密度泛函理论在化学和物理领域已经取得了巨大的成功。然而，它无法描述具有任何近似功能的自旋多重组分的能量简并性。这项工作有两个贡献。（1）我们提出了一种多态密度泛函理论（MSDFT）来表示自旋多重成分并确定多重能。MSDFT是一种混合方法，同时利用了波动函数理论和密度泛函理论。因此，基态和激发态以及不同成分的波函数，电子密度和能量密度函数在相同的基础上进行处理。在原子和分子的价态激发上说明了该方法。（2）重要的是，关键结果是，对于可以通过Kohn-Sham密度泛函理论分别确定高旋转分量的情况，可以严格定义MSDFT（描述电子耦合）中的跃迁密度泛函。可以探索数值结果，以设计和优化过渡密度函数，以用于多配置DFT中的配置耦合。