The V and B matrices used in the evaluation of δ RI are obtained from MP2-F12 calculations, and the interested reader is referred to Ref. for further details of their definition. In essence, δ RI is concerned with the difference in the V and B matrices using two different auxiliary sets; the candidate set being optimized, and a reference set (denoted by a ref superscript in Equation ), which is typically taken to be large and near-complete. Thus minimizing the value of √ δ RI for an appropriate number and composition of functions should produce an RI basis that introduces negligible errors in the approximated integrals. The difference in MP2-F12 correlation energy between a candidate RI basis and the reference basis is also of interest, and is denoted ∆RI.