Enthalpy of formation data at 298 K for 421 chemical species (Table ) are compiled from the available literature, including the Active Thermochemical Tables (ATcT), National Institute of Standards and Technology Chemistry WebBook (NIST), the Computer Analysis of Thermo Chemical data (CATCH) search and retrieval system of Pedley et al., Third Millennium Ideal Gas and Condensed Phase Thermochemical Database (3rd Mil.), the NIST Computational Chemistry Comparison and Benchmark Database (CCCBDB), and the works from Cioslowski et al. and Benson. The selected species exhibit a wide variety of chemical features, including a diversity of elements, molecular weights, charges, and multiplicities, as summarized in Table . Most species are small molecules, and about 100 species have five or more heavy atoms (Figure ), with the largest species being adamantane (C 10 H 16 ).