code: def swap_atom_keys(self, swap_dict, dict_key='atom_ids'):
          
          # Similar situation to the one from decipher_atom_keys function.
          if 'atom_ids' not in self.system.keys():
              dict_key = 'elements'
          for atom_key in range(len(self.system[dict_key])):
              for key in swap_dict.keys():
                  if self.system[dict_key][atom_key] == key:
                      self.system[dict_key][atom_key] = swap_dict[key]
docs: Swap a force field atom id for another user-defined value.
  
          This modified all values in :attr:`MolecularSystem.system['atom_ids']`
          that match criteria.
  
          This function can be used to decipher a whole forcefield if an
          appropriate dictionary is passed to the function.
  
          Example
          -------
          In this example all atom ids 'he' will be exchanged to 'H'.
  
          .. code-block:: python
  
              pywindow.molecular.MolecularSystem.swap_atom_keys({'he': 'H'})
  
          Parameters
          ----------
          swap_dict: :class:`dict`
              A dictionary containg force field atom ids (keys) to be swapped
              with corresponding values (keys' arguments).
  
          dict_key: :class:`str`
              A key in :attr:`MolecularSystem.system` dictionary to perform the
              atom keys swapping operation on. (default='atom_ids')
  
          Returns
          -------
          None : :class:`NoneType`