DESCRIPTION
The program ccenergy solves the coupled cluster singles and doubles amplitude equations. This code will work for spinrestricted closedshell HartreeFock (RHF), spinrestricted openshell HartreeFock (ROHF), spinunrestricted HartreeFock (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 33136 (2000).
 2.

A direct product decomposition approach for symmetry exploitation in
manybody 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 singledeterminant
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 BruecknerCC 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 virtualindex integrals, <abcd>,
from the atomicorbital basis integrals. This option slows
the calculation somewhat, but greatly reduces diskspace
requirements. Allowed values are NONE (default), DISK (use the AObasis
integrals stored on disk), or DIRECT (use
cints
to compute the AOintegrals 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
preprogrammed 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
fourindex quantites with up to two virtualorbital indices (e.g.,
<ijab> integrals) may be held in the cache.
 DIIS= boolean

Selects the use of the directinversion 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
BroughtonPulay completeness check used to contruct orbital domains
for localCC calculations. The default is 0.02. See J. Broughton and
P. Pulay, J. Comp. Chem. 14, 736740 (1993) and C. Hampel and
H.J. Werner, J. Chem. Phys. 104, 62866297 (1996).
 LOCAL_METHOD= string

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

Selects the desired treatment of "weak pairs" in the localCCSD
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 secondorder
perturbation theory to correct the localCCSD 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 spinadapted pair energies should be printed.
Default is false.