As kinetic modeling of HHCs has been been studied for many decades, thermochemical data for these molecules and their derivatives have accumulated through various theoretical investigations ( (1 kcal/mol). Included in many of these studies are group additivity values (GAVs), in particular the works of Bozzelli and coworkers. Since conventional group additivity does not work well for chlorocarbons, Chen and Bozzelli introduced a modified scheme which uses nonnext-nearest neighbor interaction terms (NNNI) to account for the destabilizing interaction between electronegative chlorine atoms on adjacent carbons. This modified NNNI-GAV scheme was later extended to fluorine-fluorine interactions in HFCs 12 and chlorine-oxygen interactions in chlorinated hydroperoxides. Although there are highly accurate thermochemical data in the literature, the data are very sparse and only cover a very small fraction of chemical space containing C, H, O, F, Cl, and Br chemistry. Since RMG explores a significant portion of this chemical space as it builds a mechanism, more HHC thermochemical data are needed to train RMG's group additivity schemes to improve its accuracy and reliability in estimating thermochemical properties. Thus, the focus of this work is not to more accurately calculate thermochemistry of previously studied molecules; instead, the objectives are: (1) to compute thermochemistry for a large and diverse set of halocarbons, and (2) derive an extensive ensemble of halogen GAVs with NNNIs within RMG's framework.