Strong Electron Correlation in Nitrogenase Cofactor, FeMoco.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Biochemistry

Background:

• The iron-molybdenum cofactor (FeMoco) is the catalytic site in nitrogenase responsible for converting nitrogen to ammonia.
• Understanding FeMoco's electronic structure, particularly strong electron correlation, is crucial for developing artificial nitrogen fixation catalysts.
• Previous studies predicted large active spaces ([54, 54] to [65, 57]) for accurate FeMoco electronic structure, posing computational challenges.

Purpose of the Study:

• To systematically determine the minimum active space size needed to qualitatively capture strong electron correlation in FeMoco.
• To investigate the electronic structure of related MoFe3S7 and Fe4S7 clusters.
• To assess the feasibility of using smaller active spaces for studying FeMoco's electronic properties.

Main Methods:

• Utilized Complete Active Space Self-Consistent Field (CASSCF) and 2-Reduced Density Matrix (2-RDM) methods.
• Employed active space sizes up to [14, 14] for CASSCF and [30, 30] for 2-RDM.
• Tested STO-3G, 3-21G, and DZP basis sets and analyzed fractional natural-orbital occupation numbers.

Main Results:

• Identified a competition between single-reference and multireference solutions for smaller active spaces.
• Observed a consistent multireference character in larger active spaces, indicating the necessity of multireference methods.
• Provided insights into the minimal active space requirements for qualitatively describing electron correlation in FeMoco and related clusters.

Conclusions:

• Established that smaller active spaces can qualitatively capture strong correlation in FeMoco and related clusters.
• Demonstrated the importance of multireference electronic structure methods for accurately describing FeMoco.
• This work paves the way for more computationally tractable studies of FeMoco and the design of efficient nitrogen fixation mimics.