Benchmarks for electronically excited states: time-dependent density functional theory and density functional theory based multireference configuration interaction | JoVE Visualize

Area of Science:

Computational Chemistry
Theoretical Chemistry
Quantum Chemistry

Background:

Accurate computation of electronic excited states is crucial for understanding photophysical and photochemical processes.
Previous studies have established benchmark datasets for evaluating theoretical methods.
Medium-sized organic molecules present a significant challenge for computational methods due to their complexity.

Purpose of the Study:

To evaluate the performance of time-dependent density functional theory (TD-DFT) and DFT-based multireference configuration interaction (DFT/MRCI) for calculating vertical excitation energies.
To compare TD-DFT and DFT/MRCI results against high-level ab initio calculations and established benchmark data.
To assess the accuracy of different TD-DFT functionals (BP86, B3LYP, BHLYP) for excited states.

Main Methods:

Calculations were performed on a benchmark set of 28 medium-sized organic molecules.
Vertical excitation energies, oscillator strengths, and excited-state dipole moments were computed.
Standardized geometries (MP2/6-31G(*)) and basis set (TZVP) were used for all calculations.
Results were compared against previous high-level ab initio data and reference values.

Main Results:

DFT/MRCI demonstrated the lowest mean absolute deviations for both singlet (0.22 eV) and triplet (0.24 eV) excitation energies.
TD-DFT with B3LYP functional showed moderate accuracy (0.27 eV for singlets, 0.44 eV for triplets).
TD-DFT with BP86 and BHLYP functionals were found to be significantly less accurate.
TD-DFT generally overestimated singlet states with double excitation character and underestimated triplet state energies.

Conclusions:

DFT/MRCI is a highly accurate method for calculating vertical excitation energies in medium-sized organic molecules.
TD-DFT, particularly with the B3LYP functional, offers a reasonable balance between accuracy and computational cost.
Care must be taken when interpreting TD-DFT results for singlet states with double excitation character and triplet states.