Hi Shelby!
You're passing a string as the reference file. You need to open it as a mda.Universe object and pass it to the function.
So replace the last two lines of your pcoord.py with something like this:
```
u_3BRI = mda.Universe(args.top_file, args.traj_file)
u_ref = mda.Universe(args.top_file, args.ref_file)
rmsd_dim(args.top_file, u_3BRI, u_ref)
```
Best,
Jeremy L.