A set of 421 high-accuracy reference species, compiled from multiple literature sources, serves as the training set. Details on their chemical diversity, including elemental composition, molecular size, charges, and spin states, can be found in our Part 1 work and in the Supporting Information. Each molecule's standard enthalpy of formation at 298 K is available with uncertainties typically well under 1 kcal/mol. These reference data provide the grounding for our BAC fits and model evaluations.