6.11 Non-Iterative Corrections to Coupled Cluster Energies

6.11.4 Job Control Options

(June 30, 2021)

The evaluation of a non-iterative (T) or (2) correction after a coupled-cluster singles and doubles level calculation (either CCSD, QCISD or OD) is controlled by the correlation keyword, and the specification of any frozen orbitals via N_FROZEN_CORE (and possibly N_FROZEN_VIRTUAL).

For the (2) correction, it is possible to apply the frozen core approximation in the reference coupled cluster calculation, and then correlate all orbitals in the (2) correction. This is controlled by CC_INCL_CORE_CORR, described below.

The default is to include core and core-valence correlation automatically in the CCSD(2) or OD(2) correction, if the reference CCSD or OD calculation was performed with frozen core orbitals. The reason for this choice is that core correlation is economical to include via this method (the main cost increase is only linear in the number of core orbitals), and such effects are important to account for in accurate calculations. This option should be made false if a job with explicitly frozen core orbitals is desired. One good reason for freezing core orbitals in the correction is if the basis set is physically inappropriate for describing core correlation (e.g., standard Pople basis sets, and Dunning cc-pVxZ basis sets are designed to describe valence-only correlation effects). Another good reason is if a direct comparison is desired against another method such as CCSD(T) which is always used in the same orbital window as the CCSD reference.

There are several implementations of non-iterative triples available in Q-Chem. In the original CCMAN suite, (T), (2), and (dT)/(fT) corrections can be computed. The parallel scaling of this code is very modest (4 cores max). CCMAN2 currently allows only the calculation of (T) correction for CCSD wave fucntions. By default, the CCMAN2 code is used for (T). The CCMAN code CCMAN2 is set to false. There are two versions of (T) in CCMAN2: The default version (native CCMAN2) and a new version using libpt. The implementation based on libpt is in-core MPI/OpenMP distributed-parallel. It is significantly faster in most realistic calculations (but it does not use point group symmetry, so it might show slower performance for small jobs with high symmetry). The libpt code is enabled by setting USE_LIBPT to true. The CCSD(T) calculation can be restarted using the $ccsd_pt_restart data printed during the CCSD(T) calculation. To restart the job simply copy the last printed $ccsd_pt_restart to your Q-Chem input file.

Note:  For the best performance of libpt (T) code, parallel execution should be requested, see Section 2.8.

USE_LIBPT
       Enable libpt for CCSD(T) calculations in CCMAN2.
TYPE:
       LOGICAL
DEFAULT:
       FALSE
OPTIONS:
       TRUE FALSE
RECOMMENDATION:
       libpt is now used by default in all real-valued CC/EOM-CC calculations

CC_INCL_CORE_CORR
       Whether to include the correlation contribution from frozen core orbitals in non iterative (2) corrections, such as OD(2) and CCSD(2).
TYPE:
       LOGICAL
DEFAULT:
       TRUE
OPTIONS:
       TRUE FALSE
RECOMMENDATION:
       Use the default unless no core-valence or core correlation is desired (e.g., for comparison with other methods or because the basis used cannot describe core correlation).