Checks for R2 axes perpendicular to unique axis.  For handling
        symmetric top molecules.

def _check_perpendicular_r2_axis(self, axis):
        """
        Checks for R2 axes perpendicular to unique axis.  For handling
        symmetric top molecules.
        """
        min_set = self._get_smallest_set_not_on_axis(axis)
        for s1, s2 in itertools.combinations(min_set, 2):
            test_axis = np.cross(s1.coords - s2.coords, axis)
            if np.linalg.norm(test_axis) > self.tol:
                op = SymmOp.from_axis_angle_and_translation(test_axis, 180)
                r2present = self.is_valid_op(op)
                if r2present:
                    self.symmops.append(op)
                    self.rot_sym.append((test_axis, 2))
                    return True