def opls_notation(atom_key):
<EOL>conflicts = ['<STR_LIT>', '<STR_LIT>', '<STR_LIT>']<EOL>if atom_key in conflicts:<EOL><INDENT>raise _AtomKeyConflict((<EOL>"<STR_LIT>"<EOL>"<STR_LIT>"<EOL>"<STR_LIT>"<EOL>"<STR_LIT>").format(atom_key))<EOL><DEDENT>for element in opls_atom_keys:<EOL><INDENT>if atom_key in opls_atom_keys[element]:<EOL><INDENT>return element<EOL><DEDENT><DEDENT>raise _AtomKeyError((<EOL>"<STR_LIT>").format(<EOL>atom_key))<EOL>
Return element for OPLS forcefield atom key.