def _proc_sym_top(self):
        """
        Handles symetric top molecules which has one unique eigenvalue whose
        corresponding principal axis is a unique rotational axis.  More complex
        handling required to look for R2 axes perpendicular to this unique
        axis.
        """
        if abs(self.eigvals[0] - self.eigvals[1]) < self.eig_tol:
            ind = 2
        elif abs(self.eigvals[1] - self.eigvals[2]) < self.eig_tol:
            ind = 0
        else:
            ind = 1
        logger.debug("Eigenvalues = %s." % self.eigvals)
        unique_axis = self.principal_axes[ind]
        self._check_rot_sym(unique_axis)
        logger.debug("Rotation symmetries = %s" % self.rot_sym)
        if len(self.rot_sym) > 0:
            self._check_perpendicular_r2_axis(unique_axis)

        if len(self.rot_sym) >= 2:
            self._proc_dihedral()
        elif len(self.rot_sym) == 1:
            self._proc_cyclic()
        else:
            self._proc_no_rot_sym()
Handles symetric top molecules which has one unique eigenvalue whose
        corresponding principal axis is a unique rotational axis.  More complex
        handling required to look for R2 axes perpendicular to this unique
        axis.