from the Active Thermochemical Tables (ATcT) and various literature sources. With a mean absolute error (0.83 kcal/mol) within chemical accuracy (≤ 1 kcal/mol), G4 is a suitable, relatively low cost composite quantum chemistry method for high-fidelity and high-throughput calculations of HHCs. However, for heavily halogenated systems, G4 and other composite methods do not compute enthalpies within chemical accuracy. Calculated G4 enthalpies for C 2 Cl 5 and C 2 Cl 6 in enum-halocarb4 are more than 3 kcal/mol lower than ATcT values. It is likely that heavily halogenated molecules in enum-halocarb4 have higher errors than their more sparsely halogenated counterparts. To more accurately compute thermochemistry for heavily halogenated molecules, error-cancelling reactions, such as isodesmic reactions, could be used in place of the atomization approach used in this work. However, incorporating automated error-canceling reaction generation within our workflow was beyond the scope of the present work.