Using a predefined database to span the chemical space has the advantage that all candidates fulfill certain requirements (implicitly) set upon construction of the database. For instance, they correspond to known, stable structures Figure . Cartoon depiction of different screening spaces. Random search (white) covers a wide range of candidates but includes many unphysical structures. A simple rule (green) defines a very restrictive space. Experimental or computational databases can be highly diverse but are also biased and incomplete, potentially missing entire classes of interesting materials. A formal grammar could in principle cover a large screening space without including unphysical candidates. Note that the proportions are arbitrary. In particular, the space of random structures is much larger than depicted and mostly consists of nonsensical structures.