6.8 Coupled-Cluster Methods

6.8.7 Job Control Options

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×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×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 μhartree)

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.