Determines the rotational symmetry about supplied axis.  Used only for
        symmetric top molecules which has possible rotational symmetry
        operations > 2.

def _check_rot_sym(self, axis):
        """
        Determines the rotational symmetry about supplied axis.  Used only for
        symmetric top molecules which has possible rotational symmetry
        operations > 2.
        """
        min_set = self._get_smallest_set_not_on_axis(axis)
        max_sym = len(min_set)
        for i in range(max_sym, 0, -1):
            if max_sym % i != 0:
                continue
            op = SymmOp.from_axis_angle_and_translation(axis, 360 / i)
            rotvalid = self.is_valid_op(op)
            if rotvalid:
                self.symmops.append(op)
                self.rot_sym.append((axis, i))
                return i
        return 1