Perturbative ensemble density functional theory applied to charge transfer excitations.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Charge transfer excitation energies pose challenges for standard time-dependent density functional theory (TDDFT).
• Perturbative ensemble DFT (pEDFT) is a proposed low-cost, in-principle exact alternative for excitation energy calculations.
• pEDFT has shown promise for valence excitation energies.

Purpose of the Study:

• To analytically and numerically evaluate the performance of pEDFT for charge transfer excitation energies.
• To compare pEDFT's accuracy against TDDFT in the charge transfer limit.
• To identify factors limiting pEDFT's quantitative accuracy.

Main Methods:

• Analytical examination of pEDFT in the charge transfer limit.
• Numerical calculations using the benzene-tetracyanoethylene complex.
• Comparison of pEDFT results with TDDFT benchmarks.

Main Results:

• pEDFT qualitatively captures the Mulliken limit for charge transfer excitation energies.
• pEDFT's performance shows weak dependence on the choice of density functional approximation.
• Quantitative accuracy of pEDFT is found to be lower than TDDFT.
• A novel self-interaction-like term emerges in pEDFT, negatively impacting accuracy.

Conclusions:

• pEDFT is a qualitatively useful method for charge transfer excitation energies.
• Quantitative improvements are needed to match TDDFT accuracy.
• Understanding and mitigating the new self-interaction term is crucial for advancing pEDFT.