def lbnd(self):
        """ the lower bound vector while respecting log transform

        Returns
        -------
        lbnd : pandas.Series

        """
        if not self.istransformed:
            return self.pst.parameter_data.parlbnd.copy()
        else:
            lb = self.pst.parameter_data.parlbnd.copy()
            lb[self.log_indexer] = np.log10(lb[self.log_indexer])
            return lb
the lower bound vector while respecting log transform

        Returns
        -------
        lbnd : pandas.Series