Orthogonality constrained density functional theory for electronic excited states

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Materials Science

Background:

• Density Functional Theory (DFT) is a powerful tool for electronic structure calculations.
• Calculating electronic excitation energies is crucial for understanding molecular properties and reactivity.
• Existing time-dependent DFT methods can be computationally expensive and may struggle with certain types of excitations.

Purpose of the Study:

• To present a novel time-independent variational formulation of DFT for computing electronic excitation energies.
• To develop and implement an algorithm that avoids variational collapse.
• To assess the accuracy of the proposed method for organic molecules, including charge-transfer excitations.

Main Methods:

• A time-independent variational formulation of DFT was employed.
• The excited state density functional was recast as a Kohn-Sham functional.
• An adiabatic approximation of the exchange-correlation functional was used.
• An algorithm for energy optimization with orthogonality constraints was developed and implemented.

Main Results:

• The method was benchmarked on 28 organic molecules.
• The computed excitation energies showed accuracy comparable to time-dependent DFT.
• Two novel spin-adaptation approaches were introduced, yielding similar error metrics.
• The scheme demonstrated the ability to correctly describe charge-transfer excitations.

Conclusions:

• The novel time-independent DFT scheme provides an accurate and stable method for calculating electronic excitation energies.
• The developed algorithm effectively handles orthogonality constraints without variational collapse.
• This approach offers a promising alternative to time-dependent DFT, particularly for charge-transfer excitations.