C T p ) for 6,578 noncyclic closed-shell species in enum-halocarb4. To derive HBIs, closed-shell conjugates were created by saturating radicals with a hydrogen atom. Then, the HBIs were fitted by subtracting the thermochemical properties of the radical and hydrogen atom from the thermo of the saturated conjugate species as shown in equation 1. 3,825 radicals and their saturated closed-shell conjugates from enum-halocarb4 were used to fit 76 nearest-neighbor HBIs. To improve the accuracy of the HBI scheme, 1025 second nearest-neighbor HBIs and 320 three and four member ring HBIs were derived using 6,064 noncyclic radicals and 717 cyclic radicals, respectively.