As mentioned in the Introduction, F12 calculations require an RI auxiliary basis set for the evaluation of the additional multielectron integrals, which is typically implemented using the complementary auxiliary basis set (CABS+) approach. RI auxiliary sets have been developed to specifically match the cc-pVnZ-F12 family of orbital basis sets and are commonly known as OptRI sets. They are designed to be compact and lead to a linearly independent CABS basis. That is, when a union of the OptRI and orbital basis is formed, no functions will be deleted in the CABS procedure. The exponents of the OptRI sets are optimized to minimize the value of √ δ RI for the ground state of neutral atoms, where