I looked into all par and top CHARMM files and did not find any parameters for a nitro-group. It seems that this group has not been parametrized yet. Other people asked in the forum for parameters for molecules containing this group some time ago, so I was wondering about if since then people have parametrized and/or found a way to parametrize this group.

Also, a somehow related question is: do atom types like NPP and NO2 exist in the current CHARMM force field? I cannot find such, but I may be missing some top/par files.