Simulation of heme using DFT + U: a step toward accurate spin-state energetics

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Materials Science

Background:

• Iron heme complexes are crucial in organometallic chemistry.
• Standard Density Functional Theory (DFT) methods (LDA, GGA, hybrid functionals) fail to accurately describe their low-lying electronic states, particularly magnetic splittings.
• Imidazole pentacoordinated heme is often incorrectly predicted as a triplet state by conventional DFT.

Purpose of the Study:

• To evaluate the DFT + U approach for accurately describing ligated and unligated iron heme complexes.
• To demonstrate the efficacy of DFT + U in reproducing experimental magnetic properties and molecular geometries.
• To assess DFT + U as a computationally efficient alternative to high-level quantum chemistry methods.

Main Methods:

• Investigation using the Density Functional Theory + Hubbard U (DFT + U) method.
• Systematic variation of the U parameter, focusing on values around 4 eV.
• Exploration of first-principles calculation of the U parameter via self-consistent linear-response theory.

Main Results:

• A U parameter near 4 eV yields spin transitions and molecular geometries in quantitative agreement with experimental data for iron heme complexes.
• DFT + U provides a significant improvement over standard DFT functionals for these systems.
• First-principles calculation of U showed some overestimation compared to optimal values for this specific system.

Conclusions:

• DFT + U is a viable and accurate method for studying iron porphyrin complexes.
• This approach offers a computationally reduced cost compared to correlated quantum chemistry methods.
• While first-principles U determination is promising, its application to iron heme complexes requires further refinement.