TDDFT diagnostic testing and functional assessment for triazene chromophores

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Time-dependent density functional theory (TDDFT) is widely used for calculating excitation energies.
• Generalized gradient approximation (GGA) and hybrid functionals in TDDFT can sometimes yield unreliable excitation energies.
• Orbital overlap diagnostics have been proposed to assess the reliability of TDDFT excitation energies.

Purpose of the Study:

• To evaluate a simple diagnostic test based on orbital overlap for judging TDDFT excitation energy reliability.
• To assess the performance of GGA, hybrid, and Coulomb-attenuated functionals for excitation energy calculations.
• To investigate the relationship between orbital overlap, oscillator strength, and theoretical absorption spectra.

Main Methods:

• Application of an orbital overlap diagnostic test to model systems and triazene chromophores.
• Comparison of TDDFT excitation energies with correlated ab initio calculations.
• Analysis of orbital plots to identify low- and high-overlap charge-transfer excitations.

Main Results:

• The orbital overlap test identified problematic GGA excitations but missed one involving spatially extended orbitals.
• Hybrid functionals showed no low-overlap excitations, while a GGA functional identified one.
• Coulomb-attenuated functionals provided the best agreement with ab initio results, with significantly smaller errors.

Conclusions:

• The orbital overlap diagnostic test is useful but requires refinement for certain excitation types.
• Coulomb-attenuated functionals yield high-quality excitation energies in TDDFT, comparable to ab initio methods.
• The accuracy of Coulomb-attenuated functionals may obviate the need for diagnostic tests in some cases.