: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
      * **Default**: SPECIAL

