:term:`CFOUR_CC_PROGRAM <CFOUR_CC_PROGRAM (CFOUR)>`
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      Specifies which CC program is used. The available options are VCC (=0), ECC (=1), MRCC (=2), and EXTERNAL (=3). The default for all calculations is currently VCC which requests usage of ``xvcc``, but in many cases (e.g., for CCSD and CCSD(T)) ECC should be preferred due to the better performance of ``xecc`` (available currently for CCSD, CCSD+T, CCSD(T), and closed-shell CCSDT-n, CC3, and CCSDT). MRCC and External are intended for CC programs outside the CFOUR suite, e.g., the general CC module mrcc written by M. Kallay (Budapest, Hungary). Default: VCC Note: Using the option ECC is not recommended for ROHF gradients. That is, if you are doing a geometry optimization with ROHF as your reference wave function then it is safe to use the option VCC. **PSI4 Interface:** Keyword set according to best practice for the computational method :term:`CFOUR_CALC_LEVEL <CFOUR_CALC_LEVEL (CFOUR)>`  reference :term:`CFOUR_REFERENCE <CFOUR_REFERENCE (CFOUR)>` (NYI) and derivative level :term:`CFOUR_DERIV_LEVEL <CFOUR_DERIV_LEVEL (CFOUR)>` according to Table :ref:`Best Practices <table:cfour_cc_program>` when method specified by argument to computation command (*e.g.*, when ``energy('c4-ccsd')`` requested but not when ``energy('cfour')`` requested). Value can always be set explicitly.

      * **Type**: string
      * **Possible Values**: VCC, ECC, MRCC, EXTERNAL
      * **Default**: VCC

