Charge-Transfer-Like π→π* Excitations in Time-Dependent Density Functional Theory: A Conundrum and Its Solution

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Materials Science

Background:

• Time-dependent Density Functional Theory (TDDFT) conventionally fails for charge-transfer (CT) excitations in oligoacenes.
• This failure is linked to poor spatial overlap in transformed single-electron orbitals.

Purpose of the Study:

• To resolve the TDDFT failure for CT-like excitations.
• To explain the success of range-separated hybrid functionals and propose a parameter-free approach.

Main Methods:

• Analysis of orbital spatial overlap in TDDFT.
• Application and optimal tuning of range-separated hybrid functionals.
• Validation using oligoacene and hydrocarbon systems.

Main Results:

• Identified poor orbital overlap as the root cause of TDDFT failure for CT excitations.
• Demonstrated that optimally tuned range-separated hybrid functionals, enforcing the DFT Koopmans' theorem, overcome this limitation.
• Achieved predictive accuracy without empirical parameters or external data.

Conclusions:

• The study resolves a long-standing issue in TDDFT concerning CT excitations.
• Optimally tuned range-separated hybrid functionals significantly enhance TDDFT's predictive capabilities.
• This approach broadens the applicability of TDDFT for challenging molecular systems.