Multireference and multiconfiguration ab initio methods in heme-related systems: what have we learned so far?

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Biophysical Chemistry

Background:

• Heme proteins are crucial in biological processes.
• Accurate electronic structure calculations are vital for understanding heme function.
• Multireference/multiconfiguration (MR/MC) methods offer a robust approach for complex electronic systems.

Purpose of the Study:

• To review recent applications of ab initio MR/MC methods to heme-related systems.
• To discuss the accuracy and potential errors of these methods.
• To rationalize key elements influencing MR/MC calculations for heme species.

Main Methods:

• Application of ab initio multireference/multiconfiguration (MR/MC) electronic structure methods.
• Analysis of tetra-, penta-, and hexa-coordinate heme species and high-valent iron-oxo species.
• Investigation of active space selection, including the double-shell effect.
• Utilizing localized orbitals for wave function expansion.
• Incorporating quantum mechanical/molecular mechanical (QM/MM) studies for protein environment effects.

Main Results:

• MR/MC methods show good accuracy for various heme systems, with identified systematic error sources.
• The choice of active space, particularly the double-shell effect, significantly impacts spin-state energetics and small molecule binding.
• Localized orbital expansions provide clear insights into complex electronic structures.
• Protein environment effects have been assessed in QM/MM studies.
• Comparisons with Density Functional Theory (DFT) results are presented.

Conclusions:

• Ab initio MR/MC methods are powerful tools for studying heme systems.
• Careful consideration of active space and other computational parameters is essential for reliable results.
• Future research should continue to refine these methods and explore their application to more complex biological systems.