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PROGRAM:

NAME


ccenergy - Coupled cluster singles and doubles energy program

DESCRIPTION


The program ccenergy solves the coupled cluster singles and doubles amplitude equations.
This code will work for spin-restricted closed-shell Hartree-Fock (RHF), spin-restricted
open-shell Hartree-Fock (ROHF), spin-unrestricted Hartree-Fock (UHF), and Brueckner
references. The algorithm makes use of spatial symmetry (D2h and its subgroups) and
matrix algebra techniques for efficiency. In addition, the program can be used to
simulate local correlation methods.

REFERENCES


1. An introduction to coupled cluster theory for computational chemists, T.D. Crawford
and H.F. Schaefer, Rev. Comp. Chem. 14 33-136 (2000).

2. A direct product decomposition approach for symmetry exploitation in many-body
methods. I. Energy calculations, J.F. Stanton, J. Gauss, J.D. Watts, and R.J.
Bartlett, J. Chem. Phys. 94 4334 (1991).

INPUT FORMAT


Input for this program is read from the file input.dat. The following keywords are valid:

WFN = string
Specifies the type of coupled cluster calculation desired. Valid entries at
present are CCSD, CCSD_T, BCCD, and BCCD_T. There is no default.

REFERENCE = string
Specifies the type of orbitals used for the single-determinant reference function.
Valied entries at present are RHF, UHF, and ROHF. There is no default.

MEMORY = (real MB)
The maximum amount of core memory to be used, in megabytes. See also the CACHETYPE
and CACHELEV keywords below.

CONVERGENCE = integer
Convergence desired on the CC amplitudes. Convergence is achieved when the RMS of
the error in the CC wave function is less than 10**(-n). The default is 7.

BRUECKNER_CONV = integer
Convergence desired for the orbitals in Brueckner-CC calculations. Convergence is
achieved when the largest absolute value of a single T1 ampltiudes is is less than
10**(-n). The default is 5.

MAXITER = integer
Maximum number of iterations to solve the coupled cluster equations. Defaults to
50.

FREEZE_CORE = string
Specifes whether core orbitals should be automatically frozen in the CC wave
function. If this option is set (preferably in the "default" section of input),
then the cscf program will identify the core orbitals. Subsequent codes starting
with transqt will freeze these orbitals. Note that this keyword usually makes the
additional specification of FROZEN_DOCC unnecessary, but it may still be required
in some special cases. Allowed values are true, false, yes, no, 1, 0, large and
small.

FROZEN_DOCC = integer_array
The number of lowest energy doubly occupied orbitals in each irreducible
representation from which there will be no excitations. The Cotton ordering of the
irredicible representations is used. The default is the zero vector. NB: This
keyword is actually read by the integral transformation program transqt and the
integral sorting program ccsort. See also the FREEZE_CORE keyword.

FROZEN_UOCC = integer_vector
The number of highest energy unoccupied orbitals in each irreducible representation
into which there will be no excitations. The default is the zero vector. NB: This
keyword is actually read by the integral transformation program transqt and the
integral sorting program ccsort.

PRINT = integer
Determines the verbosity of the output. A value of 0 (the default) specifies
minimal printing and a value of 2 will give output updates as each term of the CC
equations is complete. No other values have yet been used.

NUM_AMPS = integer
Specifies the number of largest T1 and T2 amplitudes to print at the end of the CC
procedure. Default is 10.

PRINT_MP2_AMPS = boolean
Indicates if the MP2 amplitudes (which are starting guesses for RHF and UHF
reference functions) should be printed. Default is false.

RESTART = boolean
Allows the program to use old T1 and T2 vectors from previous calculations as the
initial guesses for a new calculation. This is particularly useful for geometry
optimizations. The default is TRUE. NB: The ability to restart a calculation is
also controlled by the phases of the molecular orbitals. If the parameter PHASE
from the chkpt file (cf. libchkpt) is set to FALSE, then the user's RESTART input
will be ignored. This behavior can be overridden, however with the FORCE_RESTART
option.

FORCE_RESTART= boolean
Forces possible restart of the CC equations from previous T1 and T2 amplitudes.
This option should be used only by experts.

AO_BASIS= string
Compute contributions from four virtual-index integrals, <ab|cd>, from the atomic-
orbital basis integrals. This option slows the calculation somewhat, but greatly
reduces disk-space requirements. Allowed values are NONE (default), DISK (use the
AO-basis integrals stored on disk), or DIRECT (use cints to compute the AO-
integrals on the fly in each CC iteration). The DIRECT option is still
experimental and should be used only by experts.

CACHETYPE= string
Selects the priority type for maintaining the automatic memory cache used by the
DPD codes. (See libdpd.html for further details.) A value of LOW (the default)
selects a "low priority" scheme in which the deletion of items from the cache is
based on pre-programmed priorities. A value of LRU selects a "least recently used"
scheme in which the oldest item in the cache will be the first one deleted.

CACHELEV= integer
Selects the level of automatic cacheing desired in the storage of various
amplitudes, integrals, and intermediates in the coupled cluster procedure. A value
of 0 retains no quantities in cache, while a level of 6 attempts to store all
quantities in cache. For particularly large calculations, a value of 0 may help
with certain types of memory problems. The default is 2, which means that all
four-index quantites with up to two virtual-orbital indices (e.g., <ij|ab>
integrals) may be held in the cache.

DIIS= boolean
Selects the use of the direct-inversion in the iterative subspace (DIIS) technique
of Pulay for convergence acceleration. The default is TRUE.

LOCAL= boolean
Simluate the effects of local correlation techniques. The default is FALSE.

LOCAL_CUTOFF= real
The tolerance value (always between one and zero) for the Broughton-Pulay
completeness check used to contruct orbital domains for local-CC calculations. The
default is 0.02. See J. Broughton and P. Pulay, J. Comp. Chem. 14, 736-740 (1993)
and C. Hampel and H.-J. Werner, J. Chem. Phys. 104, 6286-6297 (1996).

LOCAL_METHOD= string
Selects the type of local-CCSD scheme to be simulated. Valid entries are WERNER
for the method develop by H.-J. Werner and co-workers or AOBASIS for the method
developed by G.E. Scuseria and co-workers (currently inoperative). The default is
WERNER.

LOCAL_WEAKP= string
Selects the desired treatment of "weak pairs" in the local-CCSD method. A value of
NEGLECT ignores weak pairs entirely. A value of NONE treats weak pairs in the same
manner as strong pairs. A value of MP2 (the default) uses second-order
perturbation theory to correct the local-CCSD energy computed with weak pairs
ignored.

PRINT_PAIR_ENERGIES = boolean
Indicates if the MP2 and CCSD pair energies should be printed. This is only
possible for RHF references. Default is false.

SPINADAPT_ENERGIES = boolean
Indicates if spin-adapted pair energies should be printed. Default is false.

18 June, 2002 ccenergy(1)

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