def _find_spherical_axes(self):
        """
        Looks for R5, R4, R3 and R2 axes in spherical top molecules.  Point
        group T molecules have only one unique 3-fold and one unique 2-fold
        axis. O molecules have one unique 4, 3 and 2-fold axes. I molecules
        have a unique 5-fold axis.
        """
        rot_present = defaultdict(bool)
        origin_site, dist_el_sites = cluster_sites(self.centered_mol, self.tol)
        test_set = min(dist_el_sites.values(), key=lambda s: len(s))
        coords = [s.coords for s in test_set]
        for c1, c2, c3 in itertools.combinations(coords, 3):
            for cc1, cc2 in itertools.combinations([c1, c2, c3], 2):
                if not rot_present[2]:
                    test_axis = cc1 + cc2
                    if np.linalg.norm(test_axis) > self.tol:
                        op = SymmOp.from_axis_angle_and_translation(test_axis,
                                                                    180)
                        rot_present[2] = self.is_valid_op(op)
                        if rot_present[2]:
                            self.symmops.append(op)
                            self.rot_sym.append((test_axis, 2))

            test_axis = np.cross(c2 - c1, c3 - c1)
            if np.linalg.norm(test_axis) > self.tol:
                for r in (3, 4, 5):
                    if not rot_present[r]:
                        op = SymmOp.from_axis_angle_and_translation(
                            test_axis, 360 / r)
                        rot_present[r] = self.is_valid_op(op)
                        if rot_present[r]:
                            self.symmops.append(op)
                            self.rot_sym.append((test_axis, r))
                            break
            if rot_present[2] and rot_present[3] and (
                        rot_present[4] or rot_present[5]):
                break