Static electron correlations are even more pronounced for electronically excited states relevant in photophysical and photochemical processes such as light harvesting for clean energy applications. If the frontier orbital region around the Fermi level of a given molecular structure is dense, as is the case in π-conjugated molecules or open-shell transition metal complexes (such as FeMoco), then so-called strong static electron correlation plays a decisive role already in the ground state. Although our FeMoco model is taken from the resting state, binding of a small molecule such as dinitrogen, dihydrogen, diazene, or ammonia will not decisively change the complexity of its electronic structure. We carried out (full) molecular structure optimizations with DFT methods of this FeMoco model in different charge and spin states to avoid basing our analysis on a single electronic structure. To study this bare model is no limitation, as it does not at all affect our feasibility analysis (because electrostatic QM/MM embedding will not change the number of orbitals considered for the wave function construction). 1, Right) carrying only models of the anchoring groups of the protein, which represents a suitable QM part in such calculations. Accordingly, we consider a structural model for the active site of nitrogenase ( Fig. ![]() Experiments have not yet been able to provide sufficient details on the chemical mechanism, and theoretical attempts are hampered by intrinsic methodological limitations of traditional quantum chemical methods.įor nitrogenase, an electrostatic quantum mechanical/molecular mechanical (QM/MM) model ( 26) that captures the embedding of FeMoco into the protein pocket of nitrogenase can properly account for the protein environment. Despite the importance of this process for fertilizer production that makes nitrogen from air accessible to plants, the mechanism of nitrogen fixation at FeMoco is not known. 1, Left) and the FeMoco buried in this protein ( Fig. 1 shows the MoFe protein of nitrogenase ( Fig. Mo-dependent nitrogenase consists of two subunits, the Fe protein, a homodimer, and the MoFe protein, an α 2 β 2 tetramer. Whereas the industrial Haber–Bosch catalyst requires high temperature and pressure and is therefore energy-intensive, the active site of Mo-dependent nitrogenase, the iron molybdenum cofactor (FeMoco) ( 23, 24), can split the dinitrogen triple bond at room temperature and standard pressure. This enzyme accomplishes the remarkable transformation of dinitrogen into two ammonia molecules under ambient conditions. The chemical process that we consider in this work is that of biological nitrogen fixation by the enzyme nitrogenase ( 22). For such problems on classical computers, much less than a hundred strongly correlated electrons are already out of reach for systematically improvable ab initio methods that could achieve the required accuracy. 3 and 4) this holds particularly true for molecules with many energetically close-lying orbitals. Although approximate approaches, such as density functional theory (DFT) ( 2), are very popular, their accuracy is often too low for quantitative predictions (see, e.g., refs. ![]() However, the electron correlation problem remains, despite decades of progress ( 1), one of the most vexing problems in quantum chemistry. At its core, the detailed understanding and prediction of complex reaction mechanisms then requires highly accurate electronic structure methods. As they enter exponential expressions, very accurate energy differences are required for the reliable evaluation of the rate constants. Differences of the energies of local minima and connecting transition structures determine the rates of interconversion, i.e., the chemical kinetics of the process. The relative energies of all stable structures determine the relative thermodynamical stability. Chemical reaction mechanisms are networks of molecular structures representing short- or long-lived intermediates connected by transition structures.
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