CC Section

The CC section controls CCSD and CCSD(T) coupled-cluster calculations.

Keywords

Keyword	Type	Default	Description
`Triples`	`none` \| `st` \| `pt` \| `f`	`st`	Perturbative-triples model
`FreezeCore`	boolean	false	Correlate only valence electrons
`DropVirt`	float	0.0	Drop virtual orbitals below threshold
`Conv`	integer	8	Convergence threshold = 10⁻ᴺ
`MaxIter`	integer	50	Maximum CC iterations
`Diis`	boolean	true	Enable CC-DIIS
`DiisStart`	integer	2	First iteration to use DIIS
`Shift`	float	0.0	Static level shift for amplitude update
`Read`	file	—	Read amplitudes from checkpoint
`Skip`	list	—	Comma-separated skip tokens
`Acc2E`	integer	from `INT`	ERI threshold override for CC
`Brueckner`	boolean	false	Brueckner CC (BCCD/BCCD(T))

`Triples`	Description
`none`	CCSD only (no triples)
`st`	Standard CCSD(T) (non-iterative)
`pt`	CCSD(2)_T (iterative)
`f`	Full CCSDT (iterative, expensive)

CCSD(T) uses Triples st by default.

CCSD(T) standard:

CC
  Triples   st
  FreezeCore false
END

CCSD only (no triples):

CC
  Triples  none
END

Tighter convergence:

CC
  Conv        8
  MaxIter     80
  Diis        true
  DiisStart   3
  Shift       0.2
  FreezeCore  false
END

Frozen-core CCSD(T):

CC
  Triples     st
  FreezeCore  true
END

Brueckner CCSD(T):

CC
  Triples     st
  Brueckner   true
END

CC always computes in-core TwoElectronIntegrals for the AO→MO transform, even when direct_scf is enabled for SCF.
DiisStart controls when DIIS acceleration begins (default: iteration 2).
DropVirt discards virtual orbitals with orbital energy above the threshold.
Read loads amplitudes from a checkpoint for restart.
Brueckner enables BCCD/BCCD(T) (orbital-optimized CC).