Set the DataFrame index (row labels) using one or more existing
        columns.

        Wrapper around the :meth:`pandas.DataFrame.set_index` method.

def set_index(self, keys, drop=True, append=False,
                  inplace=False, verify_integrity=False):
        """Set the DataFrame index (row labels) using one or more existing
        columns.

        Wrapper around the :meth:`pandas.DataFrame.set_index` method.
        """

        if drop is True:
            try:
                assert type(keys) is not str
                dropped_cols = set(keys)
            except (TypeError, AssertionError):
                dropped_cols = set([keys])

        if not self._required_cols <= (set(self.columns) - set(dropped_cols)):
            raise PhysicalMeaning('You drop a column that is needed to '
                                  'be a physical meaningful description '
                                  'of a molecule.')

        if inplace:
            self._frame.set_index(keys, drop=drop, append=append,
                                  inplace=inplace,
                                  verify_integrity=verify_integrity)
        else:
            new = self._frame.set_index(keys, drop=drop, append=append,
                                        inplace=inplace,
                                        verify_integrity=verify_integrity)
            return self.__class__(new, _metadata=self._metadata,
                                  metadata=self.metadata)