$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, all 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 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     A number of CCTYP choices have been superceded by more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 equations.  For example, R-CC and CR-CC were
developed prior to their CR-CCL replacement, while CR-EOML
supercedes CR-EOM.  CR-CCL provides a good approximation to
the fully iterated CCSDT method, and so is superior to the
familiar CCSD(T).  A reasonable menu is:
 
               RHF              ROHF (high spin)
               ---              ----------------
     ground states [properties]:
               CCSD [CCPRP]        CCSD [n/a]
               CCSD(T)             n/a
               CR-CCL              CR-CCL
     excited states [properties]:
               EOM-CCSD [CCPRPE]   EOM-CCSD
                  MULT=1           MULT=1,3,5 or
                                   spin-contaminated
               CR-EOML             n/a
     ionization processes:
               EA-EOM3a            n/a
               IP-EOM3a            n/a
                  MULT=2,4
 
CR-CCL =left-CCSD     + CR-CC(2,3) perturbative triples.
CR-EOML=left-EOM-CCSD + CR-CC(2,3) perturbative triples.
 
     Parallel computation 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 four 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)
 
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.)
 
 
   **** 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.
 
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.
 
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.
 
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.
 
 
   **** 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 equations (default)
       = 1 restart from AMPROCC file of a previous run
 
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 equations (default)
       = 1 restart from AMPROCC file of a previous run
 
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.
 
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