:term:`CFOUR_BASIS <CFOUR_BASIS (CFOUR)>`
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      Specifies the AO basis used in the calculation. One can either specify a basis known to CFOUR or via BASIS=SPECIAL (=0) requests an arbitrary basis (see non-standard basis-set input). However, the latter must be available in the supplied GENBAS file. As standard basis sets, currently the following are available. **Psi4 Interface:** Recommended to use instead :term:`BASIS <BASIS (MINTS)>` for larger basis set selection and greater flexibility. When :term:`BASIS <BASIS (MINTS)>` used, :term:`CFOUR_SPHERICAL <CFOUR_SPHERICAL (CFOUR)>` is set appropriately.

      * **Type**: string
      * **Possible Values**: STO-3G, 3-21G, 4-31G, 6-31G, 6-31G*, 6-31G**, 6-311G, 6-311G*, 6-311G**, DZ, DZP, TZ, TZP, TZ2P, PVDZ, PVTZ, PVQZ, PV5Z, PV6Z, PCVDZ, PCVTZ, PCVQZ, PCV5Z, PCV6Z, AUG-PVDZ, AUG-PVTZ, AUG-PVTZ, AUG-PVQZ, AUG-PV5Z, AUG-PV6Z, D-AUG-PVDZ, D-AUG-PVTZ, D-AUG-PVQZ, D-AUG-PV5Z, D-AUG-PV6Z, cc-pVDZ, cc-pVTZ, cc-pVQZ, cc-pV5Z, cc-pV6Z, cc-pCVDZ, cc-pCVTZ, cc-pCVQZ, cc-pCV5Z, cc-pCV6Z, PWCVDZ, PWCVTZ, PWCVQZ, PWCV5Z, PWCV6Z, PwCVDZ, PwCVTZ, PwCVQZ, PwCV5Z, PwCV6Z, svp, dzp, tzp, tzp2p, qz2p, pz3d2f, 13s9p4d3f, WMR, ANO0, ANO1, ANO2, EVEN\_TEMPERED, SPECIAL
      * **Default**: SPECIAL