Benchmarking quantum chemistry methods for spin-state energetics of iron complexes against quantitative experimental...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Inorganic Chemistry

Background:

• Accurate prediction of spin-state energetics is crucial for understanding iron complex behavior.
• Various quantum chemistry methods exist, but their performance for iron spin-state energetics varies.

Purpose of the Study:

• To quantitatively evaluate the accuracy of different quantum chemistry methods for predicting iron spin-state energetics.
• To establish a benchmark set of iron spin-state energetics using experimental data corrected for environmental effects.

Main Methods:

• Comparison of experimental spin-state energetics with predictions from coupled cluster theory (CCSD(T)), multiconfigurational perturbation theory (CASPT2, NEVPT2), multireference configuration interaction (MRCISD+Q), and density functional theory (DFT) methods.
• Correction of experimental data for environmental effects to create a benchmark set.

Main Results:

• Coupled cluster theory (CCSD(T)) demonstrates high accuracy with errors below 2 kcal mol⁻¹.
• CASPT2 tends to overstabilize higher-spin states; NEVPT2 performs worse.
• MRCISD+Q accuracy depends on size-consistency corrections, with Davidson-Silver and Pople performing best.
• Only a few DFT methods, like B2PLYP-D3 and OPBE, provide balanced descriptions for all studied complexes.

Conclusions:

• CCSD(T) is a highly accurate method for iron spin-state energetics.
• Multireference methods like CASPT2 and NEVPT2 have limitations.
• The performance of DFT methods is method-dependent and not universally applicable.
• Careful selection of computational methods and corrections is essential for reliable spin-state energetics predictions.