OpenMM does not give progress for minimization, just wait...
rna_composer.pdb ...
1 nm
Energy at Minima is -260640.012 kcal/mol
saved  rna_composer_min.pdb
Cmd: rna_minimize.py -sp rna_composer.pdb
Execution time: 287:15:48.840  start: 12:42:31 end: 12:58:20  
DONE!