Please be a bit more specific. CHARMM is agnostic regarding filenames, so a file that you name pretty.str, I might call ugly.pdb and they could contain the same data, to be used in the same way by CHARMM (in CHARMM you always have to provide the full filename when you read a dataset) So, is this .str file you refer to a trajectory file, an output file, a coordinate file or perhaps something else?