def _atol(self, atol):
        """Set the absolute tolerence parameter for float comparisons."""
        # NOTE: that this overrides the class value so applies to all
        # instances of the class.
        max_tol = self.__class__.MAX_TOL
        if atol < 0:
            raise QiskitError("Invalid atol: must be non-negative.")
        if atol > max_tol:
            raise QiskitError(
                "Invalid atol: must be less than {}.".format(max_tol))
        self.__class__.ATOL = atol