Returns:
            (float) Coefficient of Variation from weighted surface energy
            The ideal sphere is 0.

def anisotropy(self):
        """
        Returns:
            (float) Coefficient of Variation from weighted surface energy
            The ideal sphere is 0.
        """
        square_diff_energy = 0
        weighted_energy = self.weighted_surface_energy
        area_frac_dict = self.area_fraction_dict
        miller_energy_dict = self.miller_energy_dict

        for hkl in miller_energy_dict.keys():
            square_diff_energy += (miller_energy_dict[hkl] - weighted_energy)\
                                  ** 2 * area_frac_dict[hkl]
        return np.sqrt(square_diff_energy) / weighted_energy