CFOUR | Main / SACC Unitary Group-based Spin-adapted Coupled-cluster Calculations

The SACC module allows computing energies and first-order molecular properties for closed-shell molecules and radicals in a doublet spin state using a rigorously spin-adapted coupled-cluster method. The molecular properties include dipole and quadrupole moments, charge and spin densities at nuclei, the electric field gradient tensor and the spin-dipolar contribution to the hyperfine coupling tensor, etc. Analytic gradients for geometry optimization are not available. An SACC calculation may be performed with

CC_PROGRAM=SACC

The following calculations are possible:

Level of CC calculation	CCSD, CCSD(T)
Type of reference	RHF (closed-shell), RHF/ROHF (open-shell)^*
Type of orbitals	RHF canonical, ROHF: standard or spatial semicanonical
Type of open-shell UGA-CC ansatz	COS-CC, Normal-ordered exponential
Frozen-core calculations	Energies and only orbital-unrelaxed properties

^* For open-shell doublet states, it is possible to use the RHF orbitals for a reference state in which the
singly-occupied orbital has an extra electron compared to the target doublet state.

The relevant keywords for these calculations are:

CALC, REF
QRHF_G, QRHF_O, QRHF_S
DERIV_LEVEL, PROPS, DIFF_TYPE
IRPSING
SACC_ANSATZ (short form: SACC_A)
SACC_CALC (short form: SACC_C)
SACC_PROP (short form: SACC_P)
SACC_ORBS (short form: SACC_O)

Detailed explanations for all the above keywords are given under the "List Of Keywords In Alphabetical Order" link on the Manual page. See also the README.SACC file under cfour-top-directory/sacc for additional explanations.

The testsuite for the SACC module is found under cfour-top-directory/sacc/testsuite. The README.SACC file contains a detailed description of the example input files included in the testsuite. The input file for the COS-CCSD(T) energy calculation for the ²A₁ state of H₂O⁺ is given below as an example.

SACC input