How does DFT handle degenerate eigenvectors?

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SUMMARY

The discussion focuses on how Density Functional Theory (DFT) manages degenerate eigenstates within the framework of the Kohn-Sham equations. It highlights that the Hamiltonian in DFT is a functional of the electron density, defined as ##n(\mathbf{r})=\sum^N_{k=1}|\psi_k(\mathbf{r})|^2##. The issue arises when dealing with degenerate states, where any linear combination of these states remains a valid solution, leading to ambiguity in defining the electron density. The conversation suggests that DFT requires constraints to select a specific orientation for degenerate states, particularly in cases of rotational degeneracy, such as p orbitals.

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Mart1234
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DFT and degeneracy
I have a question about how DFT (density functional theory) handles degenerate states. The Hamiltonian in DFT is a functional of the electron density defined via ##n(\mathbf{r})=\sum^N_{k=1}|\psi_k(\mathbf{r})|^2##. However, say I have a pair of degenerate states. Then any linear combination of these two states is also a solution to the Kohn-Sham equations and the electron density based on the above definition seems to be not well defined. How does DFT address this? Is there a constraint for degenerate states which picks a proper orientation?
 
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Hm, each of the degenerate states has it's own density. E.g. if degeneracy is due to rotational degeneracy, as an example, psi might be a p orbital. As they are degenerate, the orbital can point in any direction and the corresponding electronic density will be concentrated along the same direction.
 

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