Of course, these defaults may differ between electronic structure codes, and potentially between different releases/versions of a given code. Both density fitting (DF) and resolution of the identity (RI) techniques are used for the robust approximation of electronic integrals in the majority of implementations of F12 theory. The DF and RI names are sometimes used interchangeably by some authors, but herein DF is used to signify the approximation of integrals used in the DF-HF and DF-MP2 methods, and RI is reserved for the evaluation of F12-specific integrals. This leads to F12 methods requiring three separate auxiliary basis sets (ABSs), in addition to the orbital basis set (OBS). Using the naming convention from the Molpro system of ab initio programs, these ABSs are the JKFit basis used in the evaluation of the Coulomb and exchange matrices, the MP2Fit basis used in density fitting of conventional two-electron repulsion integrals (i.e., those from the DF-MP2 method), and an RI or complementary auxiliary basis set (CABS) for the F12-specific integrals.