$TRUNCN group           (optional, relevant for RHF)
 
    This group controls the truncation of some of the
localized orbitals to just the AOs on a subset of the
atoms.  This option is particularly useful to generate
localized orbitals to be frozen when the effective
fragment potential is used to partition a system across a
chemical bond.  In other words, this group prepares the
frozen buffer zone orbitals.  This group should be used in
conjunction with RUNTYP=ENERGY (or PROP if the orbitals
are available) and either LOCAL=RUEDNBRG or BOYS, with
MOIDON set in $LOCAL.
 
DOPROJ = flag to activate MO projection/truncation, the
         default is to skip this (default=.FALSE.)
 
AUTOID = forces identification of MOs (analogous to MOIDON
         in $LOCAL).  This keyword is provided in case the
         localized orbitals are already present in $VEC,
         in which case this is a faster RUNTYP=PROP with
         LOCAL=NONE job.  Obviously, GUESS=MOREAD.
         (default=.FALSE.)
 
PLAIN  = flag to control the MO tail truncation.  A value
         of .FALSE. uses corresponding orbital projections,
         H.F.King, R.E.Stanton, H.Kim, R.E.Wyatt, R.G.Parr
         J. Chem. Phys. 47, 1936-1941(1967) and generates
         orthogonal orbitals.  A value of .TRUE. just sets
         the unwanted AOs to zero, so the resulting MOs
         need to go through the automatic orthogonalization
         step when MOREAD in the next job.
         (default=.FALSE.)
 
IMOPR  = an array specifying which MOs to be truncated. In
         most cases involving normal bonding, the options
         MOIDON or AUTOID will correctly identify all
         localized MOs belonging to the atoms in the zone
         being truncated.  However, you can inspect the
         output, and give a list of all MOs which you want
         to be truncated in this array, in case you feel
         the automatic assignment is incorrect.
         Any orbital not in the truncation set, whether
         this is chosen automatically or by IMOPR, is left
         completely unaltered.
 
                        - - -
 
There are now two ways to specify what orbitals are to
be truncated.  The most common usage is for preparation of
a buffer zone for QM/MM computations, with an Effective
Fragment Potential representing the non-quantum part of
the system.  This input is NATAB, NATBF, ICAPFR, ICAPBF,
in which case the $DATA input must be sorted into three
zones.  The first group of atoms are meant to be treated
in later runs by full quantum mechanics, the second
group by frozen localized orbitals as a 'buffer', and the
third group is to be substituted later by an effective
fragment potential (multipoles, polarizabilities, ...).
Note that in the DOPROJ=.TRUE. run, all atoms are still
quantum atoms.
 
NATAB  = number of atoms to be in the 'ab initio' zone.
 
NATBF  = number of atoms to be in the 'buffer' zone.
         The program can obtain the number of atoms in
         the remaining zone by subtraction, so it need
         not be input.
 
In case the MOIDON or AUTOID options lead to confused
assignments (unlikely in ordinary bonding situations
around the buffer zone), there are two fine tuning values.
 
ICAPFR = array indicating the identity of "capping atoms"
         which are on the border between the ab initio and
         buffer zones (in the ab initio zone).
 
ICAPBK = array indicating the identity of "capping atoms"
         which are on the border between the buffer and EFP
         zones (in the effective fragment zone).
 
See also IXCORL and IXLONE below.
 
                        - - -
 
In case truncation seems useful for some other purpose,
you can specify the atoms in any order within the $DATA
group, by the IZAT/ILAT approach.  You are supposed to
give only one of these two lists, probably whichever is
shorter:
 
IZAT   = an array containing the atoms which are NOT in
         the buffer zone.
 
ILAT   = an array containing the atoms which are in
         the buffer zone.
 
The AO coefficients of the localized orbitals present in
the buffer zone which lie on atoms outside the buffer will
be truncated.
 
See also IXCORL and IXLONE below.
 
                        - - -
 
The next two values let you remove additional orbitals
within the buffer zone from the truncation process, if that
is desirable.  These arrays can only include atoms that are
already in the buffer zone, whether this was defined by
NATBF, or IZAT/ILAT.  The default is to include all core
and lone pair orbitals, not just bonding orbitals, as the
buffer zone orbitals.
 
IXCORL = an array of atoms whose core and lone pair
         orbitals are to be considered as not belonging
         to the buffer zone orbitals.
 
IXLONE = an array of atoms for which only the lone pair
         orbitals are to be considered as not belonging
         to the buffer zone orbitals.
 
The final option controls output of the truncated orbitals
to file PUNCH for use in later runs:
 
NPUNOP =    punch out option for the truncated orbitals
       = 1  the MOs are not reordered.
       = 2  punch the truncated MOs as the first vectors
            in the $VEC MO set, with untransformed vectors
            following immediately after. (default)