Long-range correction for tight-binding TD-DFT.

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Quantum Chemistry

Background:

• Standard time-dependent density functional tight-binding approximation (TD-DFTB) struggles with charge transfer states and oscillator strengths.
• Accurate calculation of excited electronic states is vital for understanding molecular properties and designing new materials.
• Existing TD-DFTB methods often fail for systems with high conjugation, like large chromophoric polymers.

Purpose of the Study:

• To enhance the TD-DFTB method for more accurate calculations of excited electronic states.
• To improve the description of charge transfer states and oscillator strengths in TD-DFTB.
• To enable reliable electronic structure calculations for large conjugated systems using TD-DFTB.

Main Methods:

• Incorporated an exact Hartree-Fock exchange term into the ground state Hamiltonian and coupling matrix for long-range corrections (lc-TD-DFTB).
• Improved oscillator strength calculations by deriving transition dipoles from Slater-Koster files for valence orbitals.
• Tested the enhanced method on molecules known to exhibit problematic charge transfer states.

Main Results:

• The long-range corrected TD-DFTB (lc-TD-DFTB) significantly improves excitation energies for charge transfer states compared to standard TD-DFTB.
• The new method provides more realistic oscillator strengths, particularly for excitations localized on single atoms.
• Spatial overlap of orbitals predicts where standard and lc-TD-DFTB may differ significantly.

Conclusions:

• The developed lc-TD-DFTB method offers a more accurate and reliable approach for calculating excited electronic states.
• These improvements address key limitations of standard TD-DFTB, especially for charge transfer and conjugated systems.
• The enhanced method is suitable for studying the electronic structure of large chromophoric polymers.