Singlet-triplet gaps in large multireference systems: spin-flip-driven alternatives for bioinorganic modeling

Area of Science:

• Computational Chemistry
• Bioinorganic Chemistry
• Quantum Chemistry

Background:

• Accurate description of low-spin states in open-shell systems, crucial for bioinorganic chemistry, typically requires computationally expensive multireference ab initio methods.
• Existing approximations like broken-symmetry unrestricted density functional theory (DFT) have limitations in determining singlet-triplet gaps accurately.

Purpose of the Study:

• To investigate the spin-flip time-dependent DFT (SF-DFT) approach as a more efficient alternative for describing low-spin states in open-shell systems.
• To evaluate the performance of SF-DFT for calculating singlet-triplet gaps in copper-dioxygen adducts.

Main Methods:

• Employed the spin-flip time-dependent DFT (SF-DFT) methodology.
• Utilized well-documented copper-dioxygen adducts as model systems for validation.
• Compared SF-DFT results with experimental data and large-scale CASMP2 computations.

Main Results:

• SF-DFT computed singlet-triplet gaps show excellent agreement with experimental results and CASMP2 calculations.
• The SF-DFT approach enables geometry optimization at the DFT level while incorporating multireference effects.
• SF-DFT identifies electronic excitations that can guide the design of reduced variational spaces for advanced ab initio treatments.

Conclusions:

• SF-DFT provides a computationally feasible and accurate method for studying low-spin states in open-shell systems.
• This approach offers significant advantages over traditional methods, including improved efficiency and the ability to include multireference characteristics.
• SF-DFT serves as a valuable tool for both direct computation and for informing more rigorous ab initio studies in computational chemistry.