Theory of chemical bonds in metalloenzymes XXIV electronic and spin structures of FeMoco and Fe-S clusters by classical and quantum computing

Fundamental principles for theoretical understanding and elucidation of structure and reactivity of iron-sulfur (Fe-S) Fe _n S _m (n, m=2∼8) clusters are investigated and elucidated on the theoretical and experimental grounds. To this end, the nature of chemical bonds of these clusters is investigated by three methods; (1) the spin Hamiltonian model for analysis of EPR results, (2) broken-symmetry (BS) hybrid density functional theory (HDFT) methods for full geometry optimisations and elucidations of complete active spaces (CAS) for one-electron transfers reactions and (3) beyond HDFT methods such as CAS configuration interaction (CI) and MR CI for high precision energy calculations on classical and quantum computers. Theoretical concepts revealed are applied for elucidation of the mechanism of nitrogen fixation with FeMoco (Fe₇MoS₉C) cluster, indicating an important role of proton-coupled (PC) one electron spin transfer (OEST) processes instead of radical reaction mechanisms.