Spin-flip approach within time-dependent density functional tight-binding method: Theory and applications.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Standard density functional theory (DFT) and DFT-based tight-binding (DFTB) methods often struggle with electron density over-delocalization, particularly in charge-transfer systems.
• Accurate description of excited states and conical intersections is crucial for understanding photochemical reactions and molecular dynamics.

Purpose of the Study:

• To develop a spin-flip time-dependent density functional tight-binding (SF-TDDFTB) method for describing excited states.
• To extend the SF-TDDFTB method with long-range correction (SF-TDLCDFTB) to address electron density over-delocalization issues.
• To assess the accuracy and computational efficiency of the developed methods for molecular systems and minimum energy conical intersection (MECI) structures.

Main Methods:

• Development of a spin-flip time-dependent density functional tight-binding (SF-TDDFTB) approach.
• Incorporation of long-range correction (LC) into the SF-TDDFTB method, resulting in SF-TDLCDFTB.
• Numerical validation using potential energy curves for bond dissociation (HF) and double-bond rotation (ethylene), and analysis of H3 systems.
• Assessment of SF-TDDFTB and SF-TDLCDFTB for 39 S0/S1 minimum energy conical intersection (MECI) structures.

Main Results:

• SF-TDDFTB accurately reproduces smooth potential energy curves for bond dissociation and rotation.
• SF-TDLCDFTB effectively corrects electron density over-delocalization issues.
• Both SF-TDDFTB and SF-TDLCDFTB demonstrate significant reduction in computational cost for MECI structures compared to SF-TDDFT.
• The methods achieve high accuracy for MECI structures.

Conclusions:

• The developed SF-TDDFTB and SF-TDLCDFTB methods provide a computationally efficient and accurate framework for studying electronic excited states.
• These methods are particularly promising for investigating systems with charge-transfer characteristics and for locating minimum energy conical intersections.
• The long-range corrected variant addresses a key limitation of conventional DFTB and DFT methods.