Occasionally was getting an error from OpenMM that box vectors were not in the reduced form that it expects after exporting a trajectory frame into an Amber restart file and starting from that with ParmEd (perhaps natively produced restart files are already in reduced form). Interestingly, ParmEd already has the function reduce_box_vectors that is used when loading from GROMACS gro files, so I just applied that in the openmm simulations module to enforce it.