Coupled Cluster Theory

Coupled Cluster (CC) theory is a cornerstone of quantum chemistry, providing highly accurate solutions to the electronic Schrödinger equation for many-body systems. These methods offer a range of accuracy and computational cost, allowing users to choose based on their specific needs and resources.

Available Coupled Cluster Tasks

Task	Description
CCS	Coupled Cluster Singles.
CC2	Approximate Coupled Cluster Singles and Doubles.
CCSD	Coupled Cluster Singles and Doubles.
RR-CCSD	Rank-Reduced CCSD, a computationally efficient variant of CCSD.
CC3	Coupled Cluster Singles, Doubles, and approximate Triples.
CCSD(T)	CCSD with non-iterative, perturbative treatment of Triples.
CCSDT	Coupled Cluster Singles, Doubles, and Triples.
CCSDTQ	Coupled Cluster Singles, Doubles, Triples, and Quadruples.

General Keywords for CC Tasks

solver_thresh
Description	Convergence threshold for the CC solver.
Type	float
Default	1.0E-06

max_iter
Description	Maximum number of iterations allowed in the CC solver.
Type	int
Default	100

crop
Description	Whether CROP acceleration should be used in the CC solver. If False, DIIS is used.
Type	bool
Default	True

diis_max
Description	Number of CROP/DIIS vectors stored at a time in the CC solver.
Type	int
Default	10

restart
Description	Enables restarting the calculation from a checkpoint file in a .h5 format.
Type	bool
Default	True

frozencore
Description	Enables freezing of core orbitals to reduce computational cost.
Type	bool
Default	False

cholesky
Description	Enables Cholesky decomposition of AO electron repulsion integrals (ERIs).
Type	bool
Default	False
Availability	CCS CC2 CCSD

max_cholesky
Description	Upper limit of the number of Cholesky vectors allowed. In practical cases cholesky thresh defines the used amount of Cholesky vectors.
Type	float
Default	1.0E-09

cholesky_thresh
Description	Threshold for constructing Cholesky vectors.
Type	float
Default	solver_thresh / 1.0E-02

reduce_print
Description	Suppresses iteration information from the linear solver.
Type	bool
Default	True

rr-cutoff
Description	Threshold for eigenvalues in the projector for Rank-Reduced CCSD.
Type	float
Default	1.0E-4
Availability	RR-CCSD

projector_type
Description	Type of projector used in Rank-Reduced calculations.
Type	str
Default	MP2
Availability	RR-CCSD
Options	MP2 MP3