Is there anyone who can help to give the reaxff force field file on the C, Fe, H, O of the four elements under the reaction of cellulose with iron sesquioxide
Just because a ReaxFF potential is available with the atoms for your intented application, it DOES NOT mean it is transferable if the training set did not include configurations similar to your intented application. For example, there are many potentials with C/H/O/N atoms but many do not have the pi-bond parameters trained so a benzene molecule might behave in a completely unphysical manner. You need to consult the original journal article (doi links below) together with the supplementary materials to confirm the transferability of a given ReaxFF potential to your application.
That said, see table 6. ReaxFF Models — FitSNAP documentation and the potentials are in FitSNAP github: