The Crippen logP model implemented in RDKit predicts the logP value of a molecule by logP = ∑ i n i a i where n i is the number of a particular atom type i and a i is the logP contribution from that atom type. There are about 100 different atom types and their contributions were determined by fitting to experimental logP values. The atom types are defined by the nearest neighbour atoms so that, for example, the a i for C is different for ethane and methanol. However, the model is local in the sense that a i is independent of atoms not directly bonded to atom i. The a i values are taken as the ground truth for atom attributions, with the caveat that a i values from H atoms are added to the closest non-H atom.