code: def from_charmm(cls, path, positions=None, forcefield=None, strict=True, **kwargs):
          
          psf = CharmmPsfFile(path)
          if strict and forcefield is None:
              raise ValueError('PSF files require key `forcefield`.')
          if strict and positions is None:
              raise ValueError('PSF files require key `positions`.')
          psf.parmset = CharmmParameterSet(*forcefield)
          psf.loadParameters(psf.parmset)
          return cls(master=psf, topology=psf.topology, positions=positions, path=path,
                     **kwargs)
docs: Loads PSF Charmm structure from `path`. Requires `charmm_parameters`.
  
          Parameters
          ----------
          path : str
              Path to PSF file
          forcefield : list of str
              Paths to Charmm parameters files, such as *.par or *.str. REQUIRED
  
          Returns
          -------
          psf : SystemHandler
              SystemHandler with topology. Charmm parameters are embedded in
              the `master` attribute.