The reaction of the iron oxide cation with the hydrogen molecule is studied in this work using the diffusion quantum Monte Carlo (DMC) method. Previous attempts achieved a qualitative agreement between theory and experiments, but not a satisfactory quantitative explanation which concurs with the experimental facts. The DMC method provides a consistent explanation of the experimental results all along the reaction path. This is due to the fact that DMC accounts for almost all of the correlation energy, which is of vital importance for the correct description of this reaction.