Accuracy of theoretical catalysis from a model of iron-catalyzed ammonia synthesis

Density functional theory is central to the study of catalytic processes, but its accuracy is widely debated, and lack of data complicates accuracy estimates. To address these issues, this work explores a simple eight-step process of iron-catalyzed ammonia synthesis. The models’s importance lies in the availability of experimental data and the accessibility of coupled-cluster CCSD(T) calculations, enabling direct assessment of method accuracy for all reaction steps. While many functionals accurately describe the net process N₂ + 3H₂ → NH₃, errors of +100 kJ mol⁻¹ occur in many individual steps for popular functionals such as PBE, RPBE, and B3LYP, which are much worse than commonly assumed. Inclusion of the stoichiometric reaction coefficients reveals major accuracy bottlenecks surprisingly distinct from the N–N dissociation step and dependent on the applied functional. More focus should be directed to these problematic steps in order to improve the accuracy of modeling the catalytic process.