Calculating excited state properties using Kohn-Sham density functional theory.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Spectroscopy

Background:

• Accurate calculation of excited states is crucial for understanding photophysical and photochemical processes.
• Kohn-Sham density functional theory (KS-DFT) is an efficient method, but its application to excited states, particularly open-shell systems, requires careful assessment.
• The maximum overlap method (MOM) is often used in conjunction with KS-DFT for excited state calculations.

Purpose of the Study:

• To evaluate the accuracy of excited states computed using KS-DFT with MOM for adiabatic excitation energies, structures, and vibrational frequencies (harmonic and anharmonic) of open-shell singlet excited states.
• To assess the impact of post-self-consistent field spin-purification on these properties.
• To compare the performance of spin-purified KS-DFT with time-dependent density functional theory (TD-DFT).

Main Methods:

• Calculation of adiabatic excitation energies, excited state structures, and vibrational frequencies using KS-DFT with MOM.
• Application of post-self-consistent field spin-purification to the computed excited states.
• Inclusion of perturbative anharmonic corrections to harmonic vibrational frequencies.
• Comparison of results with experimental data and TD-DFT calculations.

Main Results:

• Spin-purification significantly improves KS-DFT adiabatic excitation energies, but they remain systematically lower than experimental values, with larger errors than TD-DFT.
• Spin-purification enhances the accuracy of excited state structures, achieving comparable accuracy to TD-DFT.
• Spin-purified KS-DFT yields more accurate harmonic vibrational frequencies for most modes compared to TD-DFT.
• Anharmonic corrections further improve the accuracy of vibrational frequencies, indicating a good description of the potential energy surface.

Conclusions:

• Excited state KS-DFT, especially when spin-purified, offers an efficient and accurate method for determining excited state structures and vibrational frequencies in open-shell singlet systems.
• The method shows promise for studying larger molecular systems where TD-DFT might be computationally prohibitive.
• While excitation energies require further improvement, the accuracy of structural and vibrational properties makes spin-purified KS-DFT a valuable tool in computational chemistry.