To determine how many functions of each angular symmetry to add, we first calculate δ DF and ∆DF using the cc-pVnZ-PP-F12 OBS and MP2Fit combination for the anion of the atoms Se, Te and Po. These calculations are carried out using the RICC2 module of the turbomole program, which features analytical gradients for the optimization of MP2Fit ABSs. The goal then becomes to achieve similar values of δ DF and ∆DF when the aug-cc-pVnZ-PP-F12 orbital basis is used. This is explored by first using the cc-pVnZ-PP-F12-MP2Fit ABSs and then augmenting with additional functions whose exponents are optimized to minimize δ DF . The results of this exploration exercise for the Te anion with TZ quality sets is shown in Table , where the (m -1)d electrons are treated with the frozen-core approximation.