On the calculation of charge transfer transitions with standard density functionals using constrained variational...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Standard gradient-corrected functionals in time-dependent density functional theory (TD-DFT) inaccurately describe charge transfer (CT) transitions.
• This inaccuracy is primarily attributed to the limitations of linear response theory within TD-DFT.

Purpose of the Study:

• To identify the source of TD-DFT's failure in describing charge transfer transitions.
• To develop a new theoretical framework for accurately calculating excitation energies, particularly for charge transfer phenomena.

Main Methods:

• Introduced constrained variational density functional theory (CV-DFT) as a variational approach to excitation energies.
• Investigated higher-order corrections within CV-DFT, specifically CV(2)-DFT and CV(4)-DFT.
• Developed a relaxed scheme, R-CV(4)-DFT, incorporating ground-state orbital relaxation effects.

Main Results:

• Demonstrated that linear response theory is the cause of TD-DFT's failure for CT transitions.
• Showed that including higher-order terms in CV-DFT, particularly CV(4)-DFT, yields qualitatively correct CT descriptions.
• Achieved good agreement with experimental results for charge transfer excitations using the R-CV(4)-DFT method.

Conclusions:

• The limitations of TD-DFT for charge transfer transitions stem from linear response theory.
• Constrained variational density functional theory (CV-DFT), especially at fourth order with orbital relaxation (R-CV(4)-DFT), offers a robust and accurate method for describing charge transfer excitations.