Given a structure, returns the predicted volume.
        Args:
            structure (Structure): structure w/unknown volume
            ref_structure (Structure): A reference structure with a similar
                structure but different species.
        Returns:
            a float value of the predicted volume

def predict(self, structure, ref_structure):
        """
        Given a structure, returns the predicted volume.
        Args:
            structure (Structure): structure w/unknown volume
            ref_structure (Structure): A reference structure with a similar
                structure but different species.
        Returns:
            a float value of the predicted volume
        """

        if self.check_isostructural:
            m = StructureMatcher()
            mapping = m.get_best_electronegativity_anonymous_mapping(
                structure, ref_structure)
            if mapping is None:
                raise ValueError("Input structures do not match!")

        if "ionic" in self.radii_type:
            try:
                # Use BV analyzer to determine oxidation states only if the
                # oxidation states are not already specified in the structure
                # and use_bv is true.
                if (not is_ox(structure)) and self.use_bv:
                    a = BVAnalyzer()
                    structure = a.get_oxi_state_decorated_structure(structure)
                if (not is_ox(ref_structure)) and self.use_bv:
                    a = BVAnalyzer()
                    ref_structure = a.get_oxi_state_decorated_structure(
                        ref_structure)

                comp = structure.composition
                ref_comp = ref_structure.composition

                # Check if all the associated ionic radii are available.
                if any([k.ionic_radius is None for k in list(comp.keys())]) or \
                        any([k.ionic_radius is None for k in
                             list(ref_comp.keys())]):
                    raise ValueError("Not all the ionic radii are available!")

                numerator = 0
                denominator = 0
                # Here, the 1/3 factor on the composition accounts for atomic
                # packing. We want the number per unit length.
                for k, v in comp.items():
                    numerator += k.ionic_radius * v ** (1 / 3)
                for k, v in ref_comp.items():
                    denominator += k.ionic_radius * v ** (1 / 3)

                return ref_structure.volume * (numerator / denominator) ** 3
            except Exception as ex:
                warnings.warn("Exception occured. Will attempt atomic radii.")
                # If error occurs during use of ionic radii scheme, pass
                # and see if we can resolve it using atomic radii.
                pass

        if "atomic" in self.radii_type:
            comp = structure.composition
            ref_comp = ref_structure.composition
            # Here, the 1/3 factor on the composition accounts for atomic
            # packing. We want the number per unit length.
            numerator = 0
            denominator = 0
            for k, v in comp.items():
                numerator += k.atomic_radius * v ** (1 / 3)
            for k, v in ref_comp.items():
                denominator += k.atomic_radius * v ** (1 / 3)
            return ref_structure.volume * (numerator / denominator) ** 3

        raise ValueError("Cannot find volume scaling based on radii choices "
                         "specified!")