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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)