Fig. 4: Simulation of the Fenton reaction mechanism along the intrinsic reaction coordinate (IRC).

The blue curve represents our work using an expanded (46e,26o) active space, while the green dashed curve shows previous results from ref. ⁴⁹ using a (20e, 13o) active space. The reaction pathway exhibits three distinct stages: initial formation of [Fe(H₂O)₅(H₂O₂)]²⁺ through H₂O₂ coordination to the octahedral [Fe(H₂O)₆]²⁺ complex (left molecular structure); transition state featuring O-O bond elongation (middle); and product state comprising an OH• radical and [Fe(OH)(H₂O)₅]²⁺ complex (right). Our larger active space calculations reveal a higher energy barrier (83 kcal/mol, compared with 65 kcal/mol) and more pronounced energy differences along the reaction pathway, particularly in the transition and product regions. Electronic structure calculations were performed incorporating Fe 3d and O 2s+2p shells with cc-pVTZ basis sets. Source data for the results are provided as a Source Data file.