Hartree–Fock and ground-state correlation¶

Choose the orbital reference¶

Reference	Input	When to use it
Restricted HF	RHF	Closed-shell determinants with shared spatial orbitals
Unrestricted HF	UHF	Collinear spin with independent alpha and beta orbitals
Generalized HF	GHF	Spin-orbital mixing beyond collinear UHF
Restricted open-shell HF	ROHF	Restricted spatial orbitals for open shells
Complex HF	cRHF, cUHF	Complex orbitals and dedicated response branches
Bogoliubov reference	HFB	A quasiparticle reference with anomalous density

Specify charge and multiplicity to PyDuck. Reference selection is a separate decision in input/methods. For open-shell work, record the converged determinant and the occupations used, particularly when applying maximum-overlap methods. See availability for the post-HF paths.

Perturbation theory and coupled cluster¶

MP2 and MP3 add the second- and third-order correlation contributions, respectively. Enable MP2 as well when the output should report both orders. QuAcK also exposes restricted and generalized doubles models, including ring and ladder variants. CCD and CCSD solve amplitude equations iteratively. Use the CC thresholds and iteration limits from the input reference and inspect convergence before comparing correlation energies.

The input switch CCSD(T) requests perturbative triples on top of CCSD. For comparisons across references, start with a closed-shell molecule at the same geometry, basis, orbital space, and thresholds, then change the reference.

Configuration interaction¶

Restricted CIS, CIS(D), CID, and CISD and unrestricted CIS enter the CI driver. For CIS(D), enable the CIS calculation and its doubles correction together. Select the spin blocks in the response options and check which transitions the output actually lists.