Beyond Time-Dependent Density Functional Theory Using Only Single Excitations: Methods for Computational Studies of...

Area of Science:

• Computational Quantum Chemistry
• Theoretical Spectroscopy
• Materials Science

Background:

• Time-dependent density functional theory (TDDFT) is a widely used, computationally affordable method for excited states.
• Traditional TDDFT has limitations, including incorrect potential energy surface topology near conical intersections (CIs) and high computational cost for large systems.
• These limitations hinder accurate descriptions of photochemistry and electronic processes in materials like organic semiconductors and light-harvesting complexes.

Purpose of the Study:

• To introduce novel computational methods that overcome the limitations of traditional TDDFT for excited-state calculations.
• To enable accurate modeling of photochemical processes and electronic excitations in complex molecular systems.
• To provide efficient and scalable computational tools for studying materials like organic semiconductors and light-harvesting complexes.

Main Methods:

• Development of a "spin-flip" TDDFT to rigorously correct potential energy surface topology around conical intersections.
• Implementation of a "spin-complete" TDDFT variant to ensure proper spin eigenstates.
• Creation of an ab initio Frenkel-Davydov exciton model for large systems of coupled chromophores, enabling parallelizable calculations.
• Application of excitonic approximations to TDDFT, including a "local excitation approximation" for modeling environmental effects.

Main Results:

• The spin-flip TDDFT provides a more balanced treatment of electron correlation and corrects CI topology, improving descriptions of photochemistry.
• The ab initio exciton model allows for efficient, parallelizable calculations on systems with hundreds of chromophores, including organic semiconductors.
• The local excitation approximation to TDDFT enables highly parallelizable calculations of environmental effects on localized excitations.

Conclusions:

• The developed computational methods significantly enhance the accuracy and applicability of excited-state calculations.
• These advancements facilitate the study of complex phenomena like internal conversion, intersystem crossing, and excitation energy transfer.
• The new methods offer scalable and efficient tools for investigating electronic and photophysical properties of advanced materials.