Extended N-centered ensemble density functional theory of double electronic excitations.

Area of Science:

• Quantum Chemistry
• Computational Physics
• Materials Science

Background:

• Recent advancements have unified neutral/charged ground states with neutral excited states in N-centered ensemble density functional theory (DFT).
• This extended formalism offers an exact framework for neutral and charged electronic excitations.
• The prior work revisited derivative discontinuity without relying on density's asymptotic behavior.

Purpose of the Study:

• To extend the exact ensemble DFT formalism to a broader range of electronic excitations, particularly double excitations.
• To present an exact implementation of this theory for the two-electron Hubbard dimer model.
• To compare the exact electronic structure with the ensemble DFT Kohn-Sham (KS) system.

Main Methods:

• Exact implementation of extended N-centered ensemble DFT.
• Application to the two-electron Hubbard dimer model.
• Comparative analysis of physical and KS ensemble electronic structures.

Main Results:

• Observed state inversions in the Hubbard dimer model, dependent on ensemble choice, dimer asymmetry, and correlation strength.
• Demonstrated that strong mixing of KS states can render single/double excitation classifications irrelevant.
• Provided a detailed comparison between true physical states and fictitious ensemble DFT KS states.

Conclusions:

• The extended ensemble DFT provides a robust framework for studying complex electronic excitations.
• The model highlights the nuances and potential limitations of excitation characterization in DFT.
• Further research can explore this formalism for larger, more complex molecular systems.