$DRT group           (required by MCSCF if CISTEP=GUGA)
$CIDRT group                   (required if CITYP=GUGA)
 
    This group describes the Configuration State Functions
(CSFs) used by the MCSCF or CI calculation.  The Distinct
Row Table (DRT) is the means by which the Graphical Unitary
Group Approach (GUGA) specifies configurations. The group
is spelled $DRT for MCSCF runs, and $CIDRT for CI runs.
The main difference in these is NMCC versus NFZC.
 
    There is no default for GROUP, and you must choose one
of FORS, FOCI, SOCI, or IEXCIT.
 
GROUP = the name of the point group to be used.  This is
        usually the same as that in $DATA, except for
        RUNTYP=HESSIAN, when it must be C1.  Choose from
        the following: C1, C2, CI, CS, C2V, C2H, D2, D2H,
        C4V, D4, D4H.  If your $DATA's group is not listed,
        choose only C1 here.
 
FORS  = flag specifying the Full Optimized Reaction Space
        set of configuration should be generated.  This
        is usually set true for MCSCF runs, but if it is
        not, see FORS in $MCSCF.  (Default=.FALSE.)
 
FOCI  = flag specifying first order CI.  In addition to
        the FORS configurations, all singly excited CSFs
        from the FORS reference are included.
        Default=.FALSE.
 
SOCI  = flag specifying second order CI.  In addition to
        the FORS configurations, all singly and doubly
        excited configurations from the FORS reference
        are included.  (Default=.FALSE.)
 
IEXCIT= electron excitation level, for example 2 will
        lead to a singles and doubles CI.  This variable
        is computed by the program if FORS, FOCI, or
        SOCI is chosen, otherwise it must be entered.
 
INTACT= flag to select the interacting space option.  See
        C.F.Bender, H.F.Schaefer  J.Chem.Phys. 55,
        4798-4803(1971).  The CI will include only those
        CSFs which have non-vanishing spin couplings with
        the reference configuration.  Note that when the
        Schaefer group uses this option for high spin
        ROHF references, they use Guest/Saunders orbital
        canonicalization.
 
  * * the next variables define the single reference * *
 
    The single configuration reference is defined by
filling in the orbitals by each type, in the order shown.
The default for each type is 0.
 
       Core orbitals, which are always doubly occupied:
NMCC = number of MCSCF core MOs (in $DRT only).
NFZC = number of CI frozen core MOs (in $CIDRT only).
 
       Internal orbitals, which are partially occupied:
NDOC = number of doubly occupied MOs in the reference.
NAOS = number of alpha occupied MOs in the reference,
       which are singlet coupled with a corresponding
       number of NBOS orbitals.
NBOS = number of beta spin singly occupied MOs.
NALP = number of alpha spin singly occupied MOs in the
       reference, which are coupled high spin.
NVAL = number of empty MOs in the reference.
 
       External orbitals, occupied only in FOCI or SOCI:
NEXT = number of external MOs.  If given as -1, this will
       be set to all remaining orbitals (apart from any
       frozen virtual orbitals).
NFZV = number of frozen virtual MOs, never occupied.
 
        * * the next two help with state symmetry * *
 
STSYM=  The symmetry of the electronic state.  See $DET for
        possible values: use AP/APP in Cs, not primes.
        Default is the totally symmetric representation.
 
  note: This option overwrites whatever symmetry is implied
        by NALP/NAOS/NBOS.  It is easier to pick STSYM than
        to allow its inference from the singly occupied
        orbitals, which is a relic of ancient input files.
 
NOIRR= controls labelling of the CI state symmetries.
     = 1 no labelling (default)
     = 0 usual labelling.  This can be very time consuming
         if the group is non-Abelian.
     =-1 fast labelling, in which all CSFs with small CI
         coefficients are ignored. This can produce weights
         quite different from one, due to ignoring small
         coefficients, but overall seems to work OK.
         Note that it is normal for the weights not to sum
         to 1 even for NOIRR=0 because for simplicity the
         weight determination is focused on the relative
         weights rather than absolute.  However weight do
         not sum to one only for row-mixed MOs.
     = -2,-3... fast labelling and sets SYMTOL=10**NOIRR
         for runs other than TRANSITN.  All irreps with
         weights greater than SYMTOL are considered.
 
       * * * the final choices are seldom used * * *
 
MXNINT = Buffer size for sorted integrals. (default=20000)
         Adjust this upwards if the program tells you to,
         which may occur in cases with large numbers of
         external orbitals.
 
MXNEME = Buffer size for energy matrix.  (default=10000)
 
NPRT   = Configuration printout control switch.
         This can consume a HUMUNGUS amount of paper!
         0 = no print (default)
         1 = print electron occupancies, one per line.
         2 = print determinants in each CSF.
         3 = print determinants in each CSF (for Ms=S-1).