def swap_atom_keys(self, swap_dict, dict_key='<STR_LIT>'):
<EOL>if '<STR_LIT>' not in self.system.keys():<EOL><INDENT>dict_key = '<STR_LIT>'<EOL><DEDENT>for atom_key in range(len(self.system[dict_key])):<EOL><INDENT>for key in swap_dict.keys():<EOL><INDENT>if self.system[dict_key][atom_key] == key:<EOL><INDENT>self.system[dict_key][atom_key] = swap_dict[key]<EOL><DEDENT><DEDENT><DEDENT>
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`