def get_ratio(self, max_denominator=5, index_none=None):
        """
        find the axial ratio needed for GB generator input.
        Args:
            max_denominator (int): the maximum denominator for
                the computed ratio, default to be 5.
            index_none (int): specify the irrational axis.
                0-a, 1-b, 2-c. Only may be needed for orthorombic system.
        Returns:
               axial ratio needed for GB generator (list of integers).

        """
        structure = self.initial_structure
        lat_type = self.lat_type
        if lat_type == 't' or lat_type == 'h':
            # For tetragonal and hexagonal system, ratio = c2 / a2.
            a, c = (structure.lattice.a, structure.lattice.c)
            if c > a:
                frac = Fraction(c ** 2 / a ** 2).limit_denominator(max_denominator)
                ratio = [frac.numerator, frac.denominator]
            else:
                frac = Fraction(a ** 2 / c ** 2).limit_denominator(max_denominator)
                ratio = [frac.denominator, frac.numerator]
        elif lat_type == 'r':
            # For rhombohedral system, ratio = (1 + 2 * cos(alpha)) / cos(alpha).
            cos_alpha = cos(structure.lattice.alpha / 180 * np.pi)
            frac = Fraction((1 + 2 * cos_alpha) / cos_alpha).limit_denominator(max_denominator)
            ratio = [frac.numerator, frac.denominator]
        elif lat_type == 'o':
            # For orthorhombic system, ratio = c2:b2:a2.If irrational for one axis, set it to None.
            ratio = [None] * 3
            lat = (structure.lattice.c, structure.lattice.b, structure.lattice.a)
            index = [0, 1, 2]
            if index_none is None:
                min_index = np.argmin(lat)
                index.pop(min_index)
                frac1 = Fraction(lat[index[0]] ** 2 / lat[min_index] ** 2).limit_denominator(max_denominator)
                frac2 = Fraction(lat[index[1]] ** 2 / lat[min_index] ** 2).limit_denominator(max_denominator)
                com_lcm = lcm(frac1.denominator, frac2.denominator)
                ratio[min_index] = com_lcm
                ratio[index[0]] = frac1.numerator * int(round((com_lcm / frac1.denominator)))
                ratio[index[1]] = frac2.numerator * int(round((com_lcm / frac2.denominator)))
            else:
                index.pop(index_none)
                if (lat[index[0]] > lat[index[1]]):
                    frac = Fraction(lat[index[0]] ** 2 / lat[index[1]] ** 2).limit_denominator(max_denominator)
                    ratio[index[0]] = frac.numerator
                    ratio[index[1]] = frac.denominator
                else:
                    frac = Fraction(lat[index[1]] ** 2 / lat[index[0]] ** 2).limit_denominator(max_denominator)
                    ratio[index[1]] = frac.numerator
                    ratio[index[0]] = frac.denominator
        elif lat_type == 'c':
            raise RuntimeError('Cubic system does not need axial ratio.')
        else:
            raise RuntimeError('Lattice type not implemented.')
        return ratio