code: def from_pdb(cls, path, forcefield=None, loader=PDBFile, strict=True, **kwargs):
          
          pdb = loader(path)
          box = kwargs.pop('box', pdb.topology.getPeriodicBoxVectors())
          positions = kwargs.pop('positions', pdb.positions)
          velocities = kwargs.pop('velocities', getattr(pdb, 'velocities', None))
  
          if strict and not forcefield:
              from .md import FORCEFIELDS as forcefield
              logger.info('! Forcefields for PDB not specified. Using default: %s',
                          ', '.join(forcefield))
          pdb.forcefield = ForceField(*list(process_forcefield(*forcefield)))
  
          return cls(master=pdb.forcefield, topology=pdb.topology, positions=positions,
                     velocities=velocities, box=box, path=path, **kwargs)
docs: Loads topology, positions and, potentially, velocities and vectors,
          from a PDB or PDBx file
  
          Parameters
          ----------
          path : str
              Path to PDB/PDBx file
          forcefields : list of str
              Paths to FFXML and/or FRCMOD forcefields. REQUIRED.
  
          Returns
          -------
          pdb : SystemHandler
              SystemHandler with topology, positions, and, potentially, velocities and
              box vectors. Forcefields are embedded in the `master` attribute.