$DRT

$DRT group (required for SCFTYP=MCSCF if CISTEP=GUGA)
$CIDRT group (required for CITYP=GUGA)

This group describes the -MCSCF- or -CI- wavefunction. The distinct row table is the means by which the Graphical Unitary Group Approach (GUGA) names the configurations.

The group is spelled DRT for MCSCF runs, and CIDRT for CI runs. The main difference in these is NMCC vs. 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 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.

* * 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 * *

ISTSYM = irreducible representation for GUGA wavefunction. This option overwrites whatever symmetry is implied by NALP/NAOS/NBOS. Default=0 means ISTSYM will be inferred from the symmetry of the reference, namely from the symmetry of NALP/NAOS/NBOS orbitals.
ISTSYM=    1     2     3     4     5     6     7     8
C1         A
Ci         Ag   Au
Cs         A'   A''
C2         A    B
C2v        A1   A2    B1    B2
C2h        Ag   Bu    Bg    Au
D2         A    B1    B2    B3
D2h        Ag   B1g   B2g   B3g    Au   B1u   B2u    B3u
It is no doubt easier to just select the desired ISTSYM directly. Its computation from the singly occupied orbitals is kept merely to preserve old 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 the 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 * * *

INTACT = flag to select the interacting space option. The CI will include only those spin couplings which have a
                  nonvanishing matrix element with the reference configuration.
MXNINT = Buffer size for sorted integrals. (default=20000)
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).