Density functional approximations for charge transfer excitations with intermediate spatial overlap

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Density Functional Theory (DFT) is a primary tool for electronic structure calculations.
• Time-Dependent Density Functional Theory (TDDFT) is crucial for determining spectroscopic properties of large molecules.
• The accuracy of TDDFT relies heavily on approximations for the exchange-correlation energy.

Purpose of the Study:

• To evaluate the performance of M06-2X and M08-HX density functionals in TDDFT calculations.
• To compare these functionals against CAM-B3LYP for electronic excitation energies and other molecular properties.
• To determine the suitability of M06-2X and M08-HX for exploring potential energy surfaces and excitation energies.

Main Methods:

• TDDFT calculations were performed to compute electronic excitation energies and oscillator strengths.
• The performance of M06-2X and M08-HX functionals was assessed and compared to CAM-B3LYP.
• Evaluations included charge transfer excitations, bond energies, noncovalent interactions, and reaction barrier heights.

Main Results:

• M06-2X and M08-HX functionals demonstrate comparable or superior performance to CAM-B3LYP for intermediate charge transfer excitations.
• These functionals exhibit enhanced accuracy for bond energies, noncovalent interactions, and chemical reaction barrier heights.
• The study identified specific limitations for long-range charge transfer excitations.

Conclusions:

• M06-2X and M08-HX are recommended for TDDFT studies involving potential energy surfaces and electronic excitation energies.
• These functionals offer a robust alternative for a wide range of chemical property calculations.
• Exclusion of very long-range charge transfer excitations is advised when using M06-2X and M08-HX.