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 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
overriden, 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.