Based on the storage densities predicted using the LightGBM model, we select 36,000 molecules and compute PM3 back reaction barrier heights to train and test an new Light-GBM model. There is a rough inverse correlation between storage densities and barriers to the back reaction, so the molecules are selected in such a way that all storage density ranges are represented (Figure ). It proved challenging to converge all 36,000 transition state calculations, therefore the barrier heights are estimated using adiabatic scans along with the breaking bond. A LightGBM model was trained, validated, and tested using a 83/7/10 split of the data and yielded a RMSD of 21.21 kJ/mol (Figure ). The model is used to predict the back reaction barriers of all the 421 million remaining molecules.