Different Slater exponents can be used for core-core, core-valence and valence-valence pairs as described in H.-J. Werner, G. Knizia, and F. R. Manby (Mol. Phys. 2010, DOI: 10.1080/00268976.2010.526641). The exponents are specified using the GEM_BETA option, e.g., GEM_BETA=[1.0, 1.7, 2.5] (see options, section 35.6). The three values are used for vv, cv, cc pairs, respectively. In most cases, core pairs can be defined by using the CORE directive: all orbitals that are not core and do not belong to the default valence shell are then treated as core. If part of the default valence shell is to be taken as core (e.g., the 3d shell in first-row transition metals), the core can be defined via the PRCORE variable, e.g., prcore=[4,2,2,1,4,2,2,1] for Cu. This variable must then be defined before the F12 calculation that should use it. The following example shows calculation for Br, in which the shell is treated as core.