The NWChem coupled cluster energy module is primarily the work of Alistair Rendell and Rika Kobayashi, with contributions from David Bernholdt.
The coupled cluster code can perform calculations with full iterative treatment of single and double excitations and non-iterative inclusion of triple excitation effects. It is presently limited to closed-shell (RHF) references.
Note that symmetry is not used within most of the CCSD(T) code. This can have a profound impact on performance since the speed-up from symmetry is roughly the square of the number of irreducible representations. In the absence of symmetry, the performance of this code is competitive with other programs.
The operation of the coupled cluster code is controlled by the input block
CCSD
[MAXITER <integer maxiter default 20>]
[THRESH <real thresh default 10e-6>]
[TOL2E <real tol2e default min(10e-12 , 0.01*$thresh$)>]
[DIISBAS <integer diisbas default 5>]
[FREEZE [[core] (atomic || <integer nfzc default 0>)] \
[virtual <integer nfzv default 0>]]
[IPRT <integer IPRT default 0>]
[PRINT ...]
[NOPRINT ...]
END
Note that the keyword CCSD is used for the input block
regardless of the actual level of theory desired (specified with the
TASK directive). The following directives are recognized
within the CCSD group.
The maximum number of iterations is set to 20 by default. This should be quite enough for most calculations, although particularly troublesome cases may require more.
MAXITER <integer maxiter default 20>
Controls the convergence threshold for the iterative part of the calculation. Both the RMS error in the amplitudes and the change in energy must be less than thresh.
THRESH <real thresh default 10e-6>
TOL2E <real tol2e default min(10e-12 , 0.01*$thresh$)>
The variable tol2e is used in determining the integral
screening threshold for the evaluation of the energy and related
quantities.
CAUTION! At the present time, the tol2e parameter only
affects the three- and four-virtual contributions, and the triples,
all of which are done ``on the fly''. The transformations
used for the other parts of the code currently have a hard-wired
threshold of 
. The default for tol2e is set to match
this, and since user input can only make the threshold smaller,
setting this parameter can only make calculations take longer.
Specifies the maximum size of the subspace used in DIIS convergence
acceleration. Note that DIIS requires the amplitudes and errors be
stored for each iteration in the subspace. Obviously this can
significantly increase memory requirements, and could force the user
to reduce DIISBAS for large calculations.
Measures to alleviate this problem, including more compact storage of the quantities involved, and the possibility of disk storage are being considered, but have not yet been implemented.
DIISBAS <integer diisbas default 5>
[FREEZE [[core] (atomic || <integer nfzc default 0>)] \
[virtual <integer nfzv default 0>]]
This directive is idential to that used in the MP2 module, Section 17.1.
This directive controls the level of output from the code, mostly to
facilitate debugging and the like. The larger the value, the more
output printed. From looking at the source code, the interesting
values seem to be IPRT 
5, 10, and 50.
IPRT <integer IPRT default 0>
The coupled cluster module supports the standard NWChem print control keywords, although very little in the code is actually hooked into this mechanism yet.
| Item | Print Level | Description |
| ``reference'' | high | Wavefunction information |
| ``guess pair energies'' | debug | MP2 pair energies |
| ``byproduct energies'' | default | Intermediate energies |
| ``term debugging switches'' | debug | Switches for individual terms |
Currently available methods are
CCSD - Full iterative inclusion of single and double
excitations
CCSD+T(CCSD) - The fourth order triples contribution computed with
converged singles and doubles amplitudes
CCSD(T) - The linearized triples approximation due to
Raghavachari.
The calculation is invoked using the the TASK directive, so to
perform a CCSD+T(CCSD) calculation, for example, the input file should
include the directive
TASK CCSD+T(CCSD)
Lower-level results which come as by-products (such as MP3/MP4) of the
requested calculation are generally also printed in the output file
and stored on the run-time database, but the method specified in the
TASK directive is considered the primary result.
The information in this section is intended for use by experts (both with the methodology and with the code), primarily for debugging and development work. Messing with stuff in listed in this section will probably make your calculation quantitatively wrong! Consider yourself warned!
The /DEBUG/ common block contains a number of arrays which
control the calculation of particular terms in the program. These are
15-element integer arrays (although from the code only a few elements
actually effect anything) which can be set from the input deck. See
the code for details of how the arrays are interpreted.
Printing of this data at run-time is controlled by the
"term debugging switches" print option. The values are checked against
the defaults at run-time and a warning is printed to draw attention to
the fact that the calculation does not correspond precisely to the
requested method.
DOA <integer array default 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2> DOB <integer array default 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2> DOG <integer array default 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1> DOH <integer array default 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1> DOJK <integer array default 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2> DOS <integer array default 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1> DOD <integer array default 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1>