$DIABAT group    (relevant if DIABAT=.TRUE. in $MCSCF)                          
This group controls creation of diabatic states from                            
adiabatic states using complete active space-type MCSCF                         
wavefunctions.  Diabatization is performed at a single                          
geometry, during RUNTYP=ENERGY, if DIABAT=.TRUE. in $MCSCF.                     
Diabatization is presently programmed only for CISTEP=GUGA,                     
and may be performed at either the the CASSCF level or at                       
the MCQDPT level.                                                               
Diabatization creates first the Diabatic Molecular Orbitals                     
(DMOs), and then the diabatic states by rotation of the                         
adiabatic CI states obtained with the DMOs.  DMOs are a set                     
of active orbitals evolving smoothly from a reference                           
geometry (where the adiabatic states are cleanly separated,                     
i.e. already diabatic in nature), through a region where                        
several states might come close, or even undergo avoided                        
crossings, and beyond to another region where adiabatic                         
states are once again cleanly separable.  Only one side is                      
considered the reference geometry for DMO generation.  The                      
dominant CSFs in the diabatic reference states are taken                        
from the adiabatic states at this reference geometry,                           
possibly informed by user knowledge about the dominant CSFs                     
at the other side of the crossing region.                                       
If MPLEVL=2 is specified, the diabatization program will                        
also produce diabatic states at the MCQDPT level, utilizing                     
the CAS-level DMOs during the MCQDPT diabatization.  This                       
is a simpler procedure than was used prior to 2013.  Note                       
that $MCQDPT input need not be given, as DIABAT=.TRUE. will                     
force the selection of the CSF-based MCQDPT program, and                        
will pass orbital counts and state weights from this input                      
group to the MCQDPT.                                                            
The method is described in:                                                     
H.Nakamura, D.G.Truhlar J.Chem.Phys. 115, 10353-10372(2001)                     
H.Nakamura, D.G.Truhlar J.Chem.Phys. 117, 5576-5593(2002)                       
H.Nakamura, D.G.Truhlar J.Chem.Phys. 118, 6816-6829(2003)                       
Z.H.Li, R.Valero, D.G.Truhlar                                                   
  Theoret.Chem.Acc. 118, 9-24(2007)                                             
K.R.Yang, X.Xu, D.G.Truhlar                                                     
  Chem.Phys.Lett. 573, 84-89(2013)                                              
REFMOS = a flag controlling reading of "order reference                         
         orbitals" at a reference geometry, from $DFMVEC.                       
         These are often obtained by the 3-fold way, or                         
         perhaps the 4-fold way at a nearby geometry.                           
         Default is .FALSE.                                                     
         If not set, this run will use the 3-fold way to                        
         prepare DMOs, and will also punch the DMOs for                         
         possible use as the $DFMVEC for other geometries.                      
         If set, this run will use the order reference                          
         orbitals to help align the active orbitals of the                      
         run to their order at the reference geometry.                          
         This is normally a good idea!                                          
REFGRP = a flag controlling reading of groups of CSFs                           
         expected to dominate different diabatic states.                        
         Default is .FALSE.                                                     
         If not set, for a run at some chosen reference                         
         geometry, the dominant groups are prepared and                         
         punched as a $REFCSF group.  See also SLCTTH,                          
         which sets the threshold for "dominant".                               
         If set, diabatization will be performed, using the                     
         dominant CSF group information read from $REFCSF.                      
Note: real diabatization runs must set REFGRP=.TRUE. but in                     
some cases diabatization might have no ambiguity in orbital                     
ordering, so REFMOS might be .TRUE. or .FALSE.                                  
SLCTTH = selection threshold for dominant configurations,                       
         when REFGRP=.FALSE.  Also used as a printing                           
         threshold in adiabatic and diabatic states.                            
         Default is 0.20; it pertains to CI coefficients.                       
    * * * Keywords related to state selection * * *                             
The defaults are quite reasonable, so most runs might omit                      
all of these!  Let NWEIGHT be the number of states up to                        
and including the highest weighted state in the MCSCF                           
orbital optimization (according to WSTATE in $GUGDM2).                          
NWEIGHT includes any states with zero weight below the                          
highest weighted one.  The defaults use NWEIGHT states                          
during the DMO generation, all with equal weight in the DMO                     
generation, and then diabatize NWEIGHT states.                                  
NGRST  = number of low lying states to be excluded from the                     
         final diabatization.  Default = 0.                                     
NDIAST = number of states above the first NGRST which are                       
         included in the final diabatization.                                   
         Default = NWEIGHT - NGRST.                                             
NEXST  = number of excited states above the states being                        
         dealt with.  One might set NSTATE in $GUGDIA                           
         fairly high, to monitor the position of states to                      
         ensure they don't come close in energy to the                          
         interesting states, which stop at NWEIGHT.                             
         Such extra states are ignored during generation of                     
         DMOs and during diabatization.                                         
         Default = NSTATE - NGRST - NDIAST.                                     
WBLOCK = array of three (3) weights for the GR/DIA/EX                           
         blocks of states, used during DMO generation.                          
         The default is to give all NGRST and NDIAST states                     
         equal weight in the DMO process, with no weight                        
         for the NEXST states, namely                                           
           NGRST/(NGRST+NDIAST), NDIAST/(NGRST+NDIAST), 0.0                     
Suppose the system of interest has one state lying at very                      
low energy, two excited states of interest that are close                       
to each other, and the user monitors three states higher                        
than these.  The MCSCF might very well average only the two                     
states that come close together,                                                
 $GUGDIA NSTATE=6 $END                                                          
 $GUGDM2 WSTATE(1)=0,1,1,0,0,0 $END                                             
The situation is thus NWEIGHT=3, so if no keywords are                          
chosen here, both DMO generation and diabatization involve                      
the first three states.  The most reasonable non-default                        
choice is NGRST=1, to omit the low-lying ground state from                      
the diabatization, but keep it during the DMO generation.                       
In case one also wishes to have the DMO step ignore the low                     
lying state, enter the keyword WBLOCK(1)=0.0,1.0,0.0 which                      
weights only the 2nd and 3rd states.                                            
          * * * three-fold way parameters * * *                                 
ALPHAN = weight of the state-averaged natural orbital term,                     
         default = 2.0                                                          
ALPHAR = weight of the state-specific occupation number                         
         term, default= 1.0                                                     
ALPHAT = weight of the transition density matrix term,                          
         default = 0.5                                                          
          * * * four-fold way parameters * * *                                  
The four-fold way is used if NMLAP and/or NDLAP are given.                      
NDLAP  = number of "resolution orbitals".  Resolution DMOs                      
         are introduced to determine some of DMOs in the                        
         '2' (or DOC) block, as defined by MOSLAB below.                        
         Complex multi-arrangment reactions may use this                        
         resolution to avoid scrambling certain DMOs with                       
         others of smaller occupancy.  A typical use is for                     
         systems with nearly filled lone pair orbitals.                         
         If given, $DPSVEC orbitals must be given, and if                       
         ORIENT is chosen, also $LCLDC input.                                   
         Default = 0.                                                           
NMLAP  = number of "reference orbitals" used by the                             
         maximum overlap reference MO (MORMO) criterion of                      
         the "4-fold way" method for DMO determination.                         
         Reference DMOs are obtained at some reference                          
         geometry, and advice about their selection can                         
         be gained from looking at successful applications                      
         in the literature.                                                     
         MORMO is used to the determine some of the DMOs in                     
         the '1' (or VAL) orbitals, see MOSLAB below.                           
         If given, $DIAVEC orbitals must be given, and if                       
         ORIENT is chosen, also $LCLVL input.                                   
         Default = 0.                                                           
ORIENT = logical flag to rotate the "reference orbitals"                        
         and/or "resolution orbitals" from the reference                        
         geometry to the present coordinates.                                   
         Default = .FALSE.                                                      
MOSLAB = array containing a character assessment of the                         
         active orbitals:                                                       
           DOC ('2') orbitals should have occupancies close                     
                     to two in all electronic states,                           
                     and be filled in all reference CSFs;                       
           VAL ('1') orbitals have variable occupancies                         
                     in different electronic state,                             
                     not necessarily close to 1.0, with                         
                     variable occupancy in reference CSFs;                      
           VIR ('0') orbitals should be only weakly                             
                     occupied in all electronic states,                         
                     and be empty in all reference CSFs.                        
         Only orbitals marked '2' are candidates for the                        
         "resolution orbital" step (see NDLAP), and only                        
         orbitals marked '1' are candidates for the 4-fold                      
         MORMO step (see NMLAP).  The default treats the                        
         entire active space as the VAL block:                                  
         Typically the number of '2' or '1' orbitals would                      
         exceed NDLAP and NMLAP inputs.                                         
         See also THDOC and THVIR to let the program                            
         choose the 2's, 1's, 0's based on the active                           
         orbitals occupation number.                                            
THDOC  = threshold on the state-averaged occupation numbers                     
         to identify DOC orbitals.  Default=1.8 electrons.                      
THVIR  = threshold on the state-averaged occupation numbers                     
         to identify VIR orbitals.  Default=0.2 electrons.                      
      * * * three-fold and four-fold iterations * * *                           
MAXIT  = number of D3 or MORMO Jacobi cycle iterations.                         
         The 3-fold way iterations seem very robust,                            
         but the MORMO 4-fold way iterations are less                           
         well convergent and more numerous (default=200)                        
CONVTH = threshold for convergence of D3 and MORMO.                             
         Default = 1.0E-6                                                       
                       * * *                                                    
DMOSYM = flag to allow lowering of symmetry during the DMO                      
         process.  Default = .TRUE., preserving symmetry.                       
Note: it is probably safer to enter GROUP=C1 in $DRT, and                       
NOSYM=1 in $CONTRL, and thus have symmetry off during the                       
entire run, than to choose this keyword.                                        
                       * * *                                                    
Diabatization runs can read additional input groups which                       
are not well described here.  The $REFCSF and $DFMVEC input                     
groups are often given.  They are usually prepared by a                         
three-fold DIABAT=.TRUE. run at some reference geometry,                        
using both REFGRP=.FALSE. and REFMOS=.FALSE.                                    
$REFCSF: list of dominant configurations at the reference                       
geometry, which is created by a REFGRP=.FALSE. run.  Note                       
that the SLCTTH threshold assists in deciding how many CSFs                     
are placed in the reference groups.  The $REFCSF data is                        
then read by all REFGRP=.TRUE. runs at the various other                        
geometries, in the same format as it is generated.                              
$DFMVEC: a set of temporary DMOs for ordering the DMOs at                       
the current geometry.  Usually these are DMOs prepared by a                     
3-fold way calculation at one reference geometry, but could                     
be from a 4-fold way calculation at a geometry very close                       
to the current one.  This contains only active MOS, namely                      
NDOC+NALP+NAOS+NBOS+NVAL from $DRT.                                             
$DPSVEC contains NDLAP resolution DMOs.                                         
$DIAVEC contains NMLAP reference DMOs.                                          
Note:  $DFMVEC, $DPSVEC, $DFMVEC are typical $VEC type                          
inputs.  Each is read ignoring the MO index, so you might                       
prepare the order reference MOs from converged natural                          
orbitals by simply deleting all doubly occupied orbitals,                       
and keeping all active orbitals.  Similarly, reference or                       
resolution orbitals may be plucked from any desired orbital                     
set: natural, canonical, localized...                                           
At present, there is no way for GAMESS to generate the                          
orientation information, although this can be read in.                          
Therefore their contents are not well described:                                
$LCLDC and $LCLVL are orientation data for DPSVEC and                           
DIAVEC, respectively, required if ORIENT=.TRUE.                                 

generated on 7/7/2017