The xTB Hamiltonian is using an effective coulomb-like repulsion term
where RAB is the interatomic distance and ZAB, αAB, rAB and kAB are pairwise parameters for each species pair. The pairwise parameters are usually formed from element-specific parameters to limit the actual amount of free parameters. The effective nuclear charges are formed by
Repulsion exponents are obtained as geometric mean from element specific exponents by
Finally, the exponents, rAB and kAB, of the distance dependent contributions are usually set globally for all element-pairs.