One of the key features of DFTB is the use of a two-center approximation , which requires the pairwise generation of Hamiltonian element parameters and repulsive potentials. The Foulkes-Haydock approach to the expansion of electron density in a Taylor series around a reference density gives rise to a hierarchical family of DFTB flavors, starting with the simplest version DFTB1 which is accurate to a first-order expansion, to the most involved DFTB3 flavor which contains the full third-order terms . All DFTB flavors can be cast into a spin-dependent formalism using on-site spin coupling terms . Moreover, a long-range corrected version of the second-order DFTB2 flavor has been developed in order to overcome the infamous self-interaction error inherent to conventional DFT and DFTB methods. In addition, a variety of ad-hoc chargecharge interaction damping and dispersion interactions have been introduced that can be added to account for potential deficiencies in any of the DFTB flavors . However, the performance of any of these resultant DFTB flavors are strongly dependent on the respective optimized electronic Hamiltonian parameters and repulsive potentials for the chemical element combination in question which need to be optimized for any of the DFTB flavors individually.