problem encountered when running prepare

   I encountered a problem"  Unresolved atom types in fragment" when running NWChem prepare module. I think it's because the molecule I studied are not included in the amber and charmm force field. The molecule is Epoxide. I searched online, it says I should use the mm2 force field, but I cann't find this force field in the NWChem data file. Does any one have any suggestions for me to solve this problem.