We believe that the promotion of dopant-dopant interactions to a node-level property specifically improves hetero-GNN performance by elevating the prominence of interaction features during message passing. When included as explicit nodes, the interactions are able to alter the content of the passed messages, allowing for the transmission of richer and more physically relevant information. This parallels the photophysics of UCNPs, in which energy transfer interactions between dopants critically determine the excited state populations of donors, acceptors, and Table An overview of the performance of different models on the SUNSET-1 dataset, assessed using the mean squared errors (MSE, Equation ) of predicted I vis-U V derived from 10-fold cross-validation both for SUNSET-1 in-distribution (ID) test, containing structures with up to three shells, and for SUNSET-1 out-of-distribution (OOD) test, containing structures with four shells. The first value in each cell is the mean squared error (MSE) in photon counts-per-second (cps) multiplied by 10 -3 for easier interpretation. The second value in parentheses is the normalized mean squared error (NMSE) which is the MSE normalized by the sum of squares to yield a relative error (Equation ). The lowest errors for each category, both of which are achieved by the hetero-GNN, are emphasized with bold text. their neighbors, driving nonlinear processes such as upconversion, photon avalanching, and quantum cutting. In contrast, interaction features in homo-GNNs have less influence on predictions because edge properties can only contribute to the attention score, influencing the weighting of the messages being passed between dopant nodes rather than the information contained therein. The heterogeneous graph structure also allows for the embedding of interactions between lanthanide dopants. This embedding introduces valuable inductive bias concerning the distinctness of lanthanide interactions, constraining the model to treat the dopant pairs (e.g., Yb-Er) equally, agnostic to which layers they reside in (e.g. the first or second shell), albeit with varying strengths.