The default CC module for performing correlated calculations in CFOUR is the module xvcc, as it is the most general module. However, for some type of calculations, it is advantageous to invoke the module xecc instead of xvcc. This is in particular recommended for closed-shell CCSD(T) and CCSDT calculations, as xecc involves a more rigorous spin-adaptation scheme than xvcc. In order to force a calculation to use xecc instead of xvcc, the keyword CC_PROG=ECC must be set. Additionally, single-point calculations at the closed-shell CCSD(T), CCSDT, CCSDT[Q], CCSDT(Q), and CCSDTQ levels can be performed with the xncc module (not part of the public version).
In addition, it is also possible to perform with CFOUR, CI and CC calculations via the program MRCC by M. Kallay. This is enforced by specifying CC_PROG=MRCC or CC_PROG=EXTERNAL. CFOUR then writes all files needed to run MRCC.