Nevertheless, sorting chemical formulas is a crucial preprocessing step, especially when working solely with composition for modeling. Despite the limited information, for certain structure types (e.g., Heusler's AB2C or perovskites ABO3), the indices may serve as a proxy for structure where a specific index is related to a particular structure site. However, this approach is prone to what is known as a coloring problem when indices duplicate; it might not be clear from the index which crystallographic site is occupied by which element. Commonly, when only composition features are employed, we observe nothing more than the fact that elements group according on their elemental properties, which echoes with the periodic table principle. The next level is structure maps that depict more complex information in either two or three dimensions. Figure . Approach for calculating compositional features.