Its combination of approximate quantum mechanics with empirical functions can allow for a high degree of flexibility in terms of optimization approaches, desired accuracy, and transferability across element types and diverse conditions. 12-14 DFTB models have been created for a broad range of materials, though the repulsive energy largely has been tuned to relatively low-level DFT data for condensed phases. Recent efforts have been made to enhance the accuracy of DFTB through creation of more sophisticated and systematic approaches for determining the repulsive energy term. and the Curvature Constrained Splines methodology have been used to create strictly pair-wise additive repulsive energies for several organic and inorganic systems. However, these methods can struggle for systems where greater than two-body interactions in E rep are needed. NNs have been proposed as a promising method to include many-body interactions into the DFTB repulsive energy. The resulting DFTB-NN models have the capability of predicting molecular properties for a wide range of compounds and element types. However, the drawback of utilizing NNs for E rep is similar to their use for classical force fields in that they require large amounts of training data and can have slow parameterization due to the presence of quasi-degenerate local minima, making their development exceedingly challenging.