Returns the theoretical Pauling stability ratio (rC/rA) for this environment.

def pauling_stability_ratio(self):
        """
        Returns the theoretical Pauling stability ratio (rC/rA) for this environment.
        """
        if self._pauling_stability_ratio is None:
            if self.ce_symbol in ['S:1', 'L:2']:
                self._pauling_stability_ratio = 0.0
            else:
                mindist_anions = 1000000.0
                mindist_cation_anion = 1000000.0
                for ipt1 in range(len(self.points)):
                    pt1 = np.array(self.points[ipt1])
                    mindist_cation_anion = min(mindist_cation_anion,
                                               np.linalg.norm(pt1-self.central_site))
                    for ipt2 in range(ipt1+1, len(self.points)):
                        pt2 = np.array(self.points[ipt2])
                        mindist_anions = min(mindist_anions,
                                             np.linalg.norm(pt1-pt2))
                anion_radius = mindist_anions / 2.0
                cation_radius = mindist_cation_anion - anion_radius
                self._pauling_stability_ratio = cation_radius / anion_radius
        return self._pauling_stability_ratio