Importantly, because the single point energies of isolated atoms from QM calculations often carry substantial and different systematic errors associated with the chosen level of theory, we use fitted AECs for each LOT as a replacement of direct QM calculations for isolated atoms, for we find the AEC approach to be more consistent. In this work, fitting AECs involved a linear least-squares fitting (see equations in SI) to minimize the error in calculating ∆H f (M, 298 K) for a small subset of molecules ranging across all elements: