Inter-subsystem charge-transfer excitations in exact subsystem time-dependent density-functional theory.

Area of Science:

• Quantum Chemistry
• Computational Physics

Background:

• Accurately describing charge-transfer (CT) excitations is crucial in computational chemistry.
• Existing methods often struggle with intersubsystem CT excitations.
• Projection-based embedding (PbE) offers a promising framework.

Purpose of the Study:

• To derive and validate an exact subsystem time-dependent density-functional theory (sTDDFT) for describing CT excitations within the PbE framework.
• To investigate the performance of different projection operators in PbE/sTDDFT.
• To develop efficient computational techniques for calculating CT excitation energies.

Main Methods:

• Derivation of sTDDFT working equations for PbE with various projection operators.
• Application of long-range corrected functionals and flexible basis sets.
• Development of virtual-orbital space restriction techniques.
• Calculation of electronic couplings between CT and local excitations.

Main Results:

• Supermolecular electronic excitation spectra can be fully restored using the derived exact sTDDFT.
• Both intra- and intersubsystem CT excitations are described correctly under appropriate conditions.
• Outgoing CT excitations can be calculated without intersubsystem response coupling.
• Efficient techniques significantly reduce computational cost for CT excitation energies.

Conclusions:

• The developed exact sTDDFT provides an accurate and efficient method for describing CT excitations.
• The approach enables the extraction of electronic couplings, advancing theoretical chemistry.
• This work offers a powerful tool for studying complex electronic phenomena.