In general, the xTB methods are parameterized on a chemically diverse set of molecular data. The training sets consist of relative (reactions) energies, molecular gradients, and molecular Hessians. Electronic properties, such as density matrices, partial charges, or dipole moments, were not included as reference. The energy expression of an xTB method is evaluated with a given trial parameterization to obtain energies, gradients, and Hessians in fully selfconsistent computations. The parameters are optimized by minimizing the least squares (L2) loss between the xTB computed values and accurate reference data. In the optimization, the Levenberg-Marquardt algorithm with line search is used by computing the numerical derivatives of the loss function with respect to the parameters. The parameters are initialized by hand for the core elements HCNO, or for other elements, from the row above or by interpolation between other elements.