$IRC group                     (relevant for RUNTYP=IRC)                        
    This group governs the location of the intrinsic                            
reaction coordinate (also called the minimum energy path,                       
MEP), a steepest descent path in mass weighted coordinates,                     
that connects the saddle point to reactants and products.                       
The IRC serves a proof of the mechanism for a reaction, and                     
is a starting point for reaction path dynamics.                                 
    The IRC may be found for systems with QM atoms, EFP                         
particles, or the combinations of QM and EFP particles, or                      
QM plus the optional SIMOMM plug-in MM atoms.                                   
    Restart data for RUNTYP=IRC is written into the PUNCH                       
file.  Information summarizing the reaction path is written                     
to the TRAJECT file, which should be saved, appending these                     
as various restarts are done.  The graphics program                             
MacMolPlt can display a movie of the entire mechanism, if                       
you join the entire forward and entire backwards trajectory                     
files, while changing the path distance parameter in the                        
reverse part to a negative value.                                               
----- there are five integration methods chosen by PACE.                        
PACE = GS2    selects the Gonzalez-Schlegel second order                        
              method.  This is the default method.                              
              Related input is:                                                 
  GCUT   cutoff for the norm of the mass-weighted gradient                      
         tangent (the default is chosen in the range from                       
         0.00005 to 0.00020, depending on the value for                         
         STRIDE chosen below.                                                   
  RCUT   cutoff for Cartesian RMS displacement vector.                          
         (the default is chosen in the range 0.0005 to                          
         0.0020 Bohr, depending on the value for STRIDE)                        
  ACUT   maximum angle from end points for linear                               
         interpolation (default=5 degrees)                                      
  MXOPT  maximum number of constrained optimization steps                       
         for each IRC point (default=20)                                        
  IHUPD  is the hessian update formula.  1 means Powell,                        
         2 means BFGS (default=2)                                               
  GA     is a gradient from the previous IRC point, and is                      
         used when restarting.                                                  
  OPTTOL is a gradient cutoff used to determine if the IRC                      
         is approaching a minimum.  It has the same meaning                     
         as the variable in $STATPT.  (default=0.0001)                          
PACE = LINEAR selects linear gradient following (Euler's                        
              method).  Related input is:                                       
  STABLZ switches on Ishida/Morokuma/Komornicki reaction                        
         path stabilization.  The default is .TRUE.                             
  DELTA  initial step size along the unit bisector, if                          
         STABLZ is on.  Default=0.025 Bohr.                                     
  ELBOW  is the collinearity threshold above which the                          
         stabilization is skipped.  If the mass weighted                        
         gradients at QB and QC are almost collinear, the                       
         reaction path is deemed to be curving very little,                     
         and stabilization isn't needed.  The default is                        
         175.0 degrees.  To always perform stabilization,                       
         input 180.0.                                                           
  READQB,EB,GBNORM,GB are energy and gradient data                              
         already known at the current IRC point.  If it                         
         happens that a run with STABLZ on decides to skip                      
         stabilization because of ELBOW, this data will be                      
         punched to speed the restart.                                          
PACE = QUAD   selects quadratic gradient following.                             
              Related input is:                                                 
  SAB    distance to previous point on the IRC.                                 
  GA     gradient vector at that historical point.                              
PACE = AMPC4  selects the fourth order Adams-Moulton                            
              variable step predictor-corrector.                                
              Related input is:                                                 
  GA0,GA1,GA2 which are gradients at previous points.                           
PACE = RK4    selects the 4th order Runge-Kutta variable                        
              step method.  There is no related input.                          
----- The next two are used by all PACE choices -----                           
STRIDE = Determines how far apart points on the reaction                        
         path will be.  STRIDE is used to calculate the                         
         step taken, according to the PACE you choose.                          
         The default is good for the GS2 method, which is                       
         very robust.  Other methods should request much                        
         smaller step sizes, such as 0.10 or even 0.05.                         
         (default = 0.30 sqrt(amu)-Bohr)                                        
NPOINT = The number of IRC points to be located in this                         
         run. The default is to find only the next point.                       
         (default = 1)                                                          
                   ----- constraint -----                                       
Of course, applying a constraint to the saddle point search                     
and the reaction path means that you are not locating the                       
true saddle, nor following the true reaction path.                              
IFREEZ = array of Cartesian coordinates to freeze.  The                         
         IRC stepper works in mass-weighted Cartesian                           
         space, making it impossible to freeze internal                         
         coordinates.  An input of IFREEZ(1)=4,8 means to                       
         freeze the x coordinate of the 2nd atom and the                        
         y coordinate of the 3rd atom, that is, we count                        
         coordinates x1,y1,z1,x2,y2,z2,x3,y3,z3,...                             
----- The next two let you choose your output volume -----                      
    Let F mean the first IRC point found in this run,                           
    and L mean the final IRC point of this run.                                 
    Let INTR mean the internuclear distance matrix.                             
NPRT   =  1  Print INTR at all, orbitals at all IRC points                      
          0  Print INTR at all, orbitals at F+L (default)                       
         -1  Print INTR at all, orbitals never                                  
         -2  Print INTR at F+L, orbitals never                                  
NPUN   =  1  Punch all orbitals at all IRC points                               
          0  Punch all orbitals at F+L, only occupied                           
             orbitals at IRC points between (default)                           
         -1  Punch all orbitals at F+L only                                     
         -2  Never punch orbitals                                               
----- The next two tally the reaction path results.  The                        
      defaults are appropriate for starting from a saddle                       
      point, restart values are automatically punched out.                      
NEXTPT = The number of the next point to be computed.                           
STOTAL = Total distance along the reaction path to next                         
         IRC point, in mass weighted Cartesian space.                           
----- The following controls jumping off the saddle point.                      
      If you give $HESS input, FREQ and CMODE will be                           
      generated automatically.                                                  
SADDLE = A logical variable telling if the coordinates                          
         given in the $DATA deck are at a saddle point                          
         (.TRUE.) or some other point lying on the IRC                          
         (.FALSE.).  If SADDLE is true, either a $HESS                          
         group or else FREQ and CMODE must be given.                            
         (default = .FALSE.)  Related input is:                                 
TSENGY = A logical variable controlling whether the energy                      
         and wavefunction are evaluated at the transition                       
         state coordinates given in $DATA.  Since you                           
         already know the energy from the transition state                      
         search and force field runs, the default is .F.                        
FORWRD = A logical variable controlling the direction to                        
         proceed away from a saddle point. The forward                          
         direction is defined as the direction in which                         
         the largest magnitude component of the imaginary                       
         normal mode is positive. (default =.TRUE.)                             
EVIB   = Desired decrease in energy when following the                          
         imaginary normal mode away from a saddle point.                        
         (default=0.0005 Hartree)                                               
FREQ   = The magnitude of the imaginary frequency, given                        
         in cm**-1.                                                             
CMODE  = An array of the components of the normal mode                          
         whose frequency is imaginary, in Cartesian                             
         coordinates.  Be careful with the signs!                               
   You must give FREQ and CMODE if you don't give a $HESS                       
   group, when SADDLE=.TRUE.  The option of giving these                        
   two variables instead of a $HESS does not apply to the                       
   GS2 method, which must have a hessian input, even for                        
   restarts.  Note also that EVIB is ignored by GS2 runs.                       
            * * * * * * * * * * * * * * * * * *                                 
            For hints about IRC tracking, see                                   
            the 'further information' section.                                  
            * * * * * * * * * * * * * * * * * *                                 

generated on 7/7/2017