We recommend as AO basis sets the augmented correlation consistent basis sets (denoted AVnZ) or the
specially optimized correlation consistent F12 basis sets (denoted VnZ-F12, cf. K.A. Peterson and H.-J. Werner,
J. Chem. Phys. **128**, 084102 (2008)).
Normally, triples zeta basis sets (AVTZ or VTZ-F12) yield excellent results that are close to the basis set limit.
Diffuse basis functions are rather essential both for the HF and MP2-F12 energies, and therefore the standard VTZ sets are not
recommended. If the AVnZ or VnZ-F12 orbital basis sets are used, suitable density fitting (DF) basis and resolution of
the identity (RI) basis sets are automatically chosen. For the AVnZ orbital basis sets,
AVnZ/MP2FIT amd VnZ/JKFIT basis sets are used by default for the DF and RI, respectively. The associated optimized
CABS basis set of Peterson et al. can be chosen by specifying `RI_BASIS=OPTRI`.
For the VnZ-F12 orbital basis, the associated CABS (OPTRI) basis sets are used by default.
Other basis sets can be
chosen using the `DF_BASIS`, `DF_BASIS_EXCH` and `RI_BASIS` options (cf. section 35.6).
See section 15 for more details about density fitting.

This is an example for using multiple basis sets for density fitting and resolution of the identity

The following two examples yield identical results:

In the latter example the specifications MP2FIT and JKFIT could be omitted, i.e. the input

`ccsd(t)-f12,df_basis=avtz,df_basis_exch=vtz,ri_basis=optri`

would be equivalent. In fact, since default density fitting basis sets are used

`ccsd(t)-f12,ri_basis=optri`

would be sufficient. Note, however, that without the specification OPTRI the avtz/jkfit basis set would be used for the RI. For example,

basis=avtz hf ccsd(t)-f12

is equivalent to

basis=avtz hf ccsd(t)-f12,df_basis=avtz/mp2fit,df_basis_exch=vtz/jkfit,ri_basis=vtz/jkfit

If the VnZ-F12 basis sets are used, the OPTRI sets are used by default, i.e.,

basis=vtz-f12 hf ccsd(t)-f12

is equivalent to

basis=vtz-f12 hf ccsd(t)-f12,df_basis=avtz/mp2fit,df_basis_exch=vtz/jkfit,\\ ri_basis=vtz-f12/optri

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