There are a large number of options for the coupled-cluster singles and doubles methods. They are documented in Appendix C, and, as the reader will find upon following this link, it is an extensive list indeed. Fortunately, many of them are not necessary for routine jobs. Most of the options for non-routine jobs concern altering the default iterative procedure, which is most often necessary for optimized orbital calculations (OD, QCCD), as well as the active space and EOM methods discussed later in Section 6.10. The more common options relating to convergence control are discussed there, in Section 6.10.6. Below we list the options that one should be aware of for routine calculations.

For memory options and parallel execution, see Section 6.14.

CC_CONVERGENCE

Overall convergence criterion for the coupled-cluster codes. This is designed
to ensure at least $n$ significant digits in the calculated energy, and
automatically sets the other convergence-related variables
(CC_E_CONV, CC_T_CONV, CC_THETA_CONV,
CC_THETA_GRAD_CONV) [${10}^{-n}$].

TYPE:

INTEGER

DEFAULT:

6
Energies.
7
Gradients.

OPTIONS:

$n$
Corresponding to ${10}^{-n}$ convergence criterion. Amplitude convergence is set
automatically to match energy convergence.

RECOMMENDATION:

Use the default

Note: For single point calculations, CC_E_CONV = 6 and CC_T_CONV = 4. Tighter amplitude convergence (CC_T_CONV = 5) is used for gradients and EOM calculations.

CC_DOV_THRESH

Specifies minimum allowed values for the coupled-cluster energy denominators.
Smaller values are replaced by this constant during early iterations only, so
the final results are unaffected, but initial convergence is improved when the
HOMO-LUMO gap is small or when non-conventional references are used.

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

$abcde$
Integer code is mapped to $abc\times {10}^{-de}$, *e.g.*,
$2502$ corresponds to 0.25

RECOMMENDATION:

Increase to 0.25, 0.5 or 0.75 for non convergent coupled-cluster calculations.

CC_SCALE_AMP

If not 0, scales down the step for updating coupled-cluster amplitudes in cases of problematic convergence.

TYPE:

INTEGER

DEFAULT:

0
no scaling

OPTIONS:

$abcd$
Integer code is mapped to $abcd\times {10}^{-2}$, *e.g.*,
$90$ corresponds to 0.9

RECOMMENDATION:

Use 0.9 or 0.8 for non convergent coupled-cluster calculations.

CC_MAX_ITER

Maximum number of iterations to optimize the coupled-cluster energy.

TYPE:

INTEGER

DEFAULT:

200

OPTIONS:

$n$
up to $n$ iterations to achieve convergence.

RECOMMENDATION:

None

CC_PRINT

Controls the output from post-MP2 coupled-cluster module of Q-Chem

TYPE:

INTEGER

DEFAULT:

1

OPTIONS:

$0-7$
higher values can lead to deforestation…

RECOMMENDATION:

Increase if you need more output and don’t like trees

CHOLESKY_TOL

Tolerance of Cholesky decomposition of two-electron integrals

TYPE:

INTEGER

DEFAULT:

3

OPTIONS:

$n$
Corresponds to a tolerance of ${10}^{-n}$

RECOMMENDATION:

2 - qualitative calculations, 3 - appropriate for most cases, 4 - quantitative
(error in total energy typically less than 1 $\mu $hartree)

CC_DIRECT_RI

Controls use of RI and Cholesky integrals in conventional (undecomposed) form

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

FALSE
use all integrals in decomposed format
TRUE
transform all RI or Cholesky integral back to conventional format

RECOMMENDATION:

By default all integrals are used in decomposed format allowing significant
reduction of memory use. If all integrals are transformed back (TRUE option) no
memory reduction is achieved and decomposition error is introduced, however,
the integral transformation is performed significantly faster and conventional
CC/EOM algorithms are used.