code: def opls_notation(atom_key):
      
      # warning for Ne, He, Na types overlap
      conflicts = ['ne', 'he', 'na']
      if atom_key in conflicts:
          raise _AtomKeyConflict((
              "One of the OPLS conflicting "
              "atom_keys has occured '{0}'. "
              "For how to solve this issue see the manual or "
              "MolecularSystem._atom_key_swap() doc string.").format(atom_key))
      for element in opls_atom_keys:
          if atom_key in opls_atom_keys[element]:
              return element
      # In case if atom_key was not found in the OPLS keys dictionary
      raise _AtomKeyError((
          "OPLS atom key {0} was not found in OPLS keys dictionary.").format(
              atom_key))
docs: Return element for OPLS forcefield atom key.