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 ofxecc(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., whenenergy('c4-ccsd')requested but not whenenergy('cfour')requested). Value can always be set explicitly.
- Type: string
- Possible Values: VCC, ECC, NCC, MRCC, EXTERNAL
- Default: VCC
