THERGAS calculates radicals based on the difference between the parent molecule (hydrogen added to radical) and the radical,including the disassociation energy. The changes in symmetry, vibrational moments and rotational energies are found and calculated (these effect both the entropy and the heat capacity contributions). Energetic contributions due to disassociation energies and steric energies are taken into account relative to the parent molecule.
THERGAS and THERM, in principle use the same corrections. In practice, exactly how the corrections and which corrections are taken into account differ. THERM also collects these terms in one single generic term.