if the database is constructed from experimental data. On the flipside, this means that there is a strong selection bias and the screening will not be able to discover new, unexpected materials. A rule-based definition of the library is in principle less biased, as it allows enumerating all possible structures within its constraints, not just known systems. Most catalyst screening studies typically use very simple rules, however, so that these screenings can be equally restrictive. Alternatively, one could imagine a third strategy, namely the completely random sampling of atomic arrangements. While this approach would definitely be unbiased and unconstrained, it would also lead to mostly unphysical structures, making the screening extremely inefficient.