def classify_by_composition(structures):
    """
    Classify the different IStructures by composition.
    :param structures: List of structures
    :return: Dictionary of composition strings with a List of the corresponding
    IStructures.
    """
    comp_dic = {}
    for structure in structures:
        if comp_dic.get(str(structure.composition), False):
            comp_dic[str(structure.composition)].append(structure)
        else:
            comp_dic[str(structure.composition)] = [structure, ]
    return comp_dic