$CCINP group       (optional, relevant for any CCTYP)
 
     This group controls a coupled-cluster calculation
specified by CCTYP in $CONTRL.  The reference orbitals may
be RHF or high spin ROHF.  If this group is not given, as is
often the case, only valence electrons are correlated.
Several ground state CCTYP choices obey at least a few of
the keywords from $EOMINP, so please see that group too.
Excited-state runs, such as CCTYP=EOM-CCSD, CR-EOM, or
CR-EOML, read $CCINP to define orbital ranges for the ground
state CCSD prior to generating excitations under $EOMINP's
control.
 
     A number of CCTYP choices have been superceded by more
advanced or improved approaches.  For example, R-CC and
CR-CC were developed prior to their CR-CCL replacement,
while CR-EOML supersedes CR-EOM.  CR-CCL provides an
accurate approximation to the fully iterative CCSDT method,
superior to the widely used CCSD(T), especially in bond
breaking situations.  If it is important to account for the
coupling of singly, doubly, and triply excited clusters,
which noniterative corrections to CCSD neglect, or if a
highly accurate representation of full CCSDT energetics is
desired, it is best to use the CCT3 approach.  If one is
interested in methods with up to connected triple
excitations, the most reasonable choices are:
 
               RHF              ROHF (high spin)
               ---              ----------------
 
     ground states [properties]:
               CCSD [CCPRP]        CCSD [n/a]
               CCSD(T)             n/a
               CR-CCL              CR-CCL
               CCSD3A              CCSD3A
               CCT3                CCT3
 
     excited states [properties]:
               EOM-CCSD [CCPRPE]   EOM-CCSD
               CR-EOML             n/a
 
     electron-attached and ionized states:
               EA-EOM2             not recommended
               IP-EOM2             not recommended
               EA-EOM3A            not recommended
               IP-EOM3A            not recommended
 
CR-CCL   = CCSD + left-CCSD + CR-CC(2,3) triples corrections.
CCSD3A   = Active-space CCSDt calculation. When all orbitals
           are active, CCSDt becomes CCSDT.
CCT3     = CCSDt + CC(P;Q)-based moment correction due to a
           subset of triples missing in CCSDt.
 
CR-EOML  = EOM-CCSD + left-EOM-CCSD + CR-EOMCC(2,3) triples
           corrections (CR-EOMCCSD(T) corrections are printed
           as well).
 
EA-EOM3A = EA-EOMCCSD(3p2h) calculation with an active space
           treatment of 3p2h excitations.
IP-EOM3A = IP-EOMCCSD(3h2p) calculation with an active space
           treatment of 3h2p excitations.
 
When reading CR-EOM and CR-EOML outputs, it is recommended
to focus on DELTA-CR-EOMCCSD(T) excitation energies and
size-intensivity corrected delta-CR-EOMCC(2,3) total and
excitation energies.
 
     Parallel computation using DDI is possible for RHF
references only and only for CCTYP=CCSD or CCSD(T).  Memory
use in parallel runs is exotic: use EXETYP=CHECK with PARALL
in $SYSTEM set on prints the per node memory requirements.
 
     See the "Further Information" section of this manual
for more details about coupled-cluster runs.
 
 
   **** The first six pertain to both RHF and ROHF ****
 
NCORE  = gives the number of frozen core orbitals to be
         omitted from the CC calculation.  The default
         is the number of chemical core orbitals.
 
NFZV   = the number of frozen virtual orbitals to be
         omitted from the calculation.  (default is 0)
 
MAXCC  = defines the maximum number of CCSD (or LCCD, CCD)
         iterations.  This parameter also applies to ROHF's
         left CC vector solver, but not RHF's left vector.
         See MAXCCL for RHF.  (default=30 for LCCD, CCD,
         and CCSD, and 50 for CCSD3A)
 
ICONV  = defines the convergence criterion for the cluster
         amplitudes, as 10**(-ICONV).  The ROHF reference
         also uses this for its left eigenstate solver, but
         see CVGEOM in $EOMINP for RHF references.
         (default is 7, but it tightens to 8 for FMO-CC.)
 
NACTO  = number of active occupied orbitals to be used
         in CCSDt (CCTYP=CCSD3A) and CC(t;3) (CCTYP=CCT3)
         calculations. This number corresponds to the beta
         active occupied orbitals. The alpha active
         occupied orbitals are automatically calculated
         according to the formula (MULT - 1) + NACTO.
 
NACTU  = number of active unoccupied orbitals to be used
         in CCSDt (CCTYP=CCSD3A) and CC(t;3) (CCTYP=CCT3)
         calculations. This number corresponds to the alpha
         active unnocupied orbitals. The beta active
         unoccupied orbitals are automatically calculated
         according to the formula (MULT - 1) + NACTU.
 
 
   **** the next group pertains to RHF reference only ****
 
CCPRP  = a flag to select computation of the CCSD level
         ground state density matrix (see also CCPRPE in
         $EOMINP for EOM-CCSD level excited states).  The
         computation takes significant extra time, to
         obtain left eigenstates, so the default is .FALSE.
         except for CCTYP=CR-CCL or CR-EOML, where the work
         required for properties must be done anyway.
         This keyword is only available in serial runs.
 
Notes: CCSD is the only level at which properties can be
obtained.  Therefore this option can only be chosen for
CCTYP=CCSD, CR-CCL, EOM-CCSD, CR-EOM, or CR-EOML.  A CCSD
run requesting CCPRP=.TRUE. will internally change itself to
EOM-CCSD to run the left CCSD, but since NSTATE of $EOMINP
will still be zero, this remains a ground state calculation.
Note that the convergence criterion for left eigenstates is
CVGEOM in $EOMINP, which is set to obtain excitation
energies, and may need tightening.  There is little reason
to select any of these:
 
MAXCCL = iteration limit on the left eigenstate needed by
         CCSD properties, or CR-CCL energies.
         This is just a synonym for MAXEOM in $EOMINP.
         If you want to alter the left state's convergence
         tolerance, use CVGEOM in $EOMINP.  The right CCSD
         state's convergence is set by MAXCC and ICONV.
 
NWORD  = a limit on memory to be used in the CC steps.
         The default is 0, meaning all memory available
         will be used.  Note: This variable is not used by
         CCSD3A.
 
IREST  = defines the restart option.  If the value of IREST
         is greater or equal 3, program will restart from
         the earlier CC run.  This requires saving the disk
         file CCREST from the previous CC run.  Values of
         IREST between 0 and 3 should not be used.  In
         general, the value of IREST is used by the program
         to set the iteration counter in the restarted run.
         The default is 0, meaning no restart is attempted.
         IREST does not apply to CCSD3A and CCT3 runs using
         SCFTYP=RHF, which are automatically redirected to
         SCFTYP=ROHF as long as SCFTYP=RHF and MULT=1.  The
         CCSD3A and CCT3 runs are controlled by KREST and
         LREST options described below, independent of
         SCFTYP.
 
MXDIIS = defines the number of cluster amplitude vectors
         from previous iterations to be included in the
         DIIS extrapolation during the CCSD (or LCCD, CCD)
         iterative process.  The default value of MXDIIS is
         5 for all but small problems.  The DIIS solver can
         be disengaged by entering MXDIIS = 0.  It is not
         necessary to change the default value of MXDIIS,
         unless the CC equations do not converge in spite
         of increasing the value of MAXCC.  MXDIIS does not
         apply to CCSD3A iterations.
 
AMPTSH = defines a threshold for eliminating small cluster
         amplitudes from the CC calculations.  Amplitudes
         with absolute values smaller than AMPTSH are set
         to zero.  The default is to retain all small
         amplitudes, meaning fully accurate CC iterations.
         Default = 0.0.  AMPTSH does not apply to CCSD3A
         iterations.
 
CCERI =  defines whether or not a tensor decomposition
         technique should be applied to the two-electron
         repulsion integrals (2ERIs) for CC calculations.
      =  STANDARD, no tensor decomposition, the standard
         4-center 2ERIs are used (default).
      =  RI, the resolution-of-the-identity approximation
         is employed and the 4-center 2ERIs are assembled
         as products of 3-center RI integrals on the fly.
         The RI approximation largely eliminates memory
         bottlenecks of CC calculations and offers a better
         parallel scaling and CPU usage.
 
 Don't forget to see also the $RICC input group for
 additional parameters.
 
   **** the next group pertains to ROHF reference only ****
        There is little reason to select any of these.
 
MULT   = spin multiplicity to use in the reference
         determinant during the CC computation.  The value
         of MULT given in the $CONTRL input determines the
         spin state for the ROHF orbital optimization, and
         is the default for the CC.  It would be quite
         unusual to use a different spin in the SCF than in
         the CC.  The MULT keyword in $EOMINP is of greater
         physical interest.
 
IOPMET = method for the CR-CC(2,3) triples correction.
       = 0 means try 1 and then try 2 (default)
       = 1, the high memory option
         This option uses the most memory, but the least
         disk storage and the least CPU time.
       = 2, the high disk option
         This option uses least memory, by storing a large
         disk file.  Time is slightly more than IOPMET=1,
         but the disk file is (NO**3 * NU**3)/6 words,
         where NO = correlated orbitals, and NU= virtuals.
       = 3, the high I/O option
         This option requires slightly more memory than 2,
         and slightly more disk than 1, but does much I/O.
         It is also the slowest of the three choices.
Check runs will print memory needed by all three options.
 
KREST  = 0 fresh start of the CCSD or CCSDt iterations
         (default)
       = 1 restart from AMPROCC ($JOB.F70) file of a
         previous CCSD run, or from $JOB.MOE file of a
         previous CCSDt run when CCTYP=CCSD3A or CCT3.
 
KMICRO = n performs DIIS extrapolation of the open shell
         CCSD, every n iterations (default is 6)
         Enter 0 to avoid using the DIIS converger.
 
LREST  = 0 fresh start of the left CCSD iterations, or
         the left CCSD-like iterations using singly
         and doubly excited cluster amplitudes obtained
         in CCSDt calculations when CCTYP=CCT3 (default)
       = 1 restart from AMPROCC ($JOB.F70) file of a
         previous left CCSD (CCTYP=CR-CCL) or left
         CCSD-like (CCTYP=CCT3) run.
         LREST=1 must be combined with KREST=1.
 
LMICRO = n performs DIIS extrapolation of the open shell
         left equations, every n iterations (default is 5)
         Enter 0 to avoid using the DIIS converger.
         KMICRO and LMICRO are ignored for trivial
         problem sizes.
 
   **** The IOSIZE input parameter applies to all CC ****
        calculations.
 
IOSIZE = n enables "chunking" of direct access I/O during
         CC computations.  The direct access (DA) record
         size of several scratch files generated during CC
         computations can become quite large, but many
         compilers will not permit DA record lengths in excess
         of 2GB.  If the record size L of a DA file exceeds n,
         then each record will be written to/read from disk
         in multiple chunks, each of which is at most n bytes
         in length.  For example, if the logical record length
         L = 2,400,000,000 and n = 1,000,000,000, then each
         logical record will be processed in 3 chunks, with
         respective record lengths of 10^9, 10^9, and 4^8 bytes,
         thereby avoiding the 2GB record length limit.
 
       = 0 disables I/O chunking (default.)
         An exetyp=check job with iosize=0 will indicate if
         chunking needs to be enabled and will also list
         recommended values of n.