The Smooth Overlap of Atomic Positions (SOAP) descriptors were used as the initial node features in Equation (2). SOAP describes the local environment around each atom in a molecule using a basis of radial functions and spherical harmonics. There are several parameters that need to be specified to compute SOAP descriptors: the element type that will be examined, the maximum number of radial functions n max , the maximum number of of spherical harmonics l max , the cutoff value for the local region around the atom r cut , and the width (standard deviation) of the Gaussians used to expand the atomic density σ . A previous study tried to find optimal values for these parameters, and arrived at recommendations of around n max = 9 and l max = 9. However, we found that using values for n max and l max which differed slightly from these did not impact on the performance of the GNN and enabled more rapid convergence of the GNN parameters during training. The SOAP parameters used in this work were n max = 12, l max = 6, r cut = 5.0 Å, and σ = 0.5 Å. In addition, we added a categorical feature to indicate the hybridization state of the atom (none, sp, sp2, or sp3; encoded as integers). This information was read from the CCDC database for the crystal structures examined. The inclusion of this feature was found to improve model performance. The elements that were examined were B, Br, C, Cl, F, H, I, N, O, P, S, Se, and Si.