# CFOUR_CC_PROGRAM

> 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 CFOUR_CALC_LEVEL reference CFOUR_REFERENCE (NYI) and derivative level CFOUR_DERIV_LEVEL according to Table Best Practices 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