“…Inspired by the relevance of detecting and counting graph substructures in applications, we propose to understand the power of GNN architectures via the substructures that they can and cannot count. Also referred to by various names including graphlets, motifs, subgraphs and graph fragments, graph substructures are well-studied and relevant for graph-related tasks in computational chemistry (Deshpande et al, 2002;Murray and Rees, 2009;Duvenaud et al, 2015;Jin et al, 2018Jin et al, , 2019Jin et al, , 2020, computational biology (Koyutrk et al, 2004) and social network studies (Jiang et al, 2010). In organic chemistry, for example, certain patterns of atoms called functional groups are usually considered indicative of the molecules' properties (Lemke, 2003;Pope et al, 2018).…”