Approximately Self-Consistent Ensemble Density Functional Theory: Toward Inclusion of All Correlations

Area of Science:

• Quantum Chemistry
• Computational Physics
• Theoretical Chemistry

Background:

• Ensemble density functional theory (EDFT) is a developing area with potential for excited state calculations.
• Accurate treatment of density-driven correlations and effective potentials are key challenges for EDFT.

Purpose of the Study:

• To introduce simple theories for density-driven correlations and effective potentials within EDFT.
• To evaluate the performance of EDFT against other methods for calculating low-lying excitation energies.

Main Methods:

• Development of novel theoretical approaches for density-driven correlations and effective potentials in EDFT.
• Comparison of EDFT with ΔSCF DFT and time-dependent DFT using identical density functional approximations.
• Testing on small atoms and molecules to determine low-lying energy gaps.

Main Results:

• The proposed EDFT theories successfully address density-driven correlations and effective potentials.
• EDFT demonstrated superior performance compared to ΔSCF DFT and time-dependent DFT for low-lying gaps in most tested systems.
• The findings hold even when employing the same underlying density functional approximations across all methods.

Conclusions:

• EDFT is established as a viable and promising tool for cost-effective studies of low-lying excited states.
• A clear pathway for practical implementation of arbitrary functional approximations in EDFT is provided.
• The developed theories offer a significant advancement in applying EDFT to excited-state problems.