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Laqua, Henryk; Kussmann, Jörg; Ochsenfeld, Christian (2018): Communication: Density functional theory model for multi-reference systems based on the exact-exchange hole normalization. In: Journal of Chemical Physics, Vol. 148, No. 12, 121101
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The correct description of multi-reference electronic ground states within Kohn-Sham density functional theory (DFT) requires an ensemble-state representation, employing fractionally occupied orbitals. However, the use of fractional orbital occupation leads to non-normalized exact-exchange holes, resulting in large fractional-spin errors for conventional approximative density functionals. In this communication, we present a simple approach to directly include the exact-exchange-hole normalization into DFT. Compared to conventional functionals, our model strongly improves the description for multi-reference systems, while preserving the accuracy in the single-reference case. We analyze the performance of our proposed method at the example of spin-averaged atoms and spin-restricted bond dissociation energy surfaces. Published by AIP Publishing.