• Applicable to molecules, chemical reactions, clusters, materials In addition to the general points in this table, we add specific comments for chemical reactions. These are challenging owing to numerous stationary points and fragment channels, as noted in our recent Perspective examining the MD17 dataset. A technical point that has not received much attention in the MLP literature is the need for the PES to describe the various fragments channels. There are two important aspects of importance. First, the PES should precisely describe the fragments, which themselves may be polyatomic molecules, and rigorously separate so that these fragments are non-interacting at large distance. Second, and related to the first, the PES should describe the long-range interaction between the fragments. We note that PIP-based potentials dominate the field of gas-phase chemical reactions. A review of such potentials appeared in 2011, and development to ever-more complex reactive potentials has been significant. We give a sample of recent PIP-based reactive potentials in Fig. and also in as inputs for Gaussian Process Regression. 22? These will be briefly reviewed below.