Single-shot compositional optimization of lowdensity HEAs. The design of low-density alloys has attracted significant attention in structural materials applications, 36 so we utilize our GNN modeling to demonstrate whether one can explain an experimentally inaccessible target 10 (the single-phase FCC solid solution of equimolar AlLiMgSnZn) and improve its thermodynamic synthesizability via compositional optimization. First, Model 1 (from Figure ) was tested by predicting formation energies ( Êf,M1 ) of 10,000 random equimolar Al-LiMgSnZn and off-equimolar Al 0.5 LiMg 0.5 SnZn 0.5 configurations. This screening both converges the probability distribution of formation energies and predicts significantly out-of-train-distribution values, especially for Al 0.5 LiMg 0.5 SnZn 0.5 (Figure ). We re-validate a sampling of these configurations (Figure ) to generally demonstrate both the overall model accuracy (MAE = 5.2 and 7.9 meV/atom for AlLiMgSnZn and Al 0.5 LiMg 0.5 SnZn 0.5 , respectively) and the accuracy in extrapolating to out-of-train-distribution formation energies.