$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.                              
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.                                      
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                         
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)                              

generated on 7/7/2017