$GRADEX group  (optional, for RUNTYP=GRADEXTR)                                  
   This group controls the gradient extremal following                          
algorithm.  The GEs leave stationary points parallel to                         
each of the normal modes of the hessian.  Sometimes a GE                        
leaving a minimum will find a transition state, and thus                        
provides us with a way of finding that saddle point.  GEs                       
have many unusual mathematical properties, and you should                       
be aware that they normally differ a great deal from IRCs.                      
   The search will always be performed in cartesian                             
coordinates, but internal coordinates along the way may                         
be printed by the usual specification of NZVAR and $ZMAT.                       
METHOD = algorithm selection.                                                   
         SR   A predictor-corrector method due to Sun                           
              and Ruedenberg (default).                                         
         JJH  A method due to Jorgensen, Jensen and                             
NSTEP  = maximum number of predictor steps to take.                             
DPRED  = the stepsize for the predictor step.                                   
         (default = 0.10)                                                       
STPT   = a flag to indicate whether the initial geometry                        
         is considered a stationary point. If .TRUE.,                           
         the geometry will be perturbed by STSTEP along                         
         the IFOLOW normal mode.                                                
         (default = .TRUE.)                                                     
STSTEP = the stepsize for jumping away from a stationary                        
         point. (default = 0.01)                                                
IFOLOW = Mode selection option.  (default is 1)                                 
         If STPT=.TRUE., the initial geometry will be                           
         perturbed by STSTEP along the IFOLOW normal mode.                      
         Note that IFOLOW can be positive or negative,                          
         depending on the direction the normal mode                             
         should be followed in. The positive direction                          
         is defined as the one where the largest component                      
         of the Hessian eigenvector is positive.                                
         If STPT=.FALSE. the sign of IFOLOW determines                          
         which direction the GE is followed in. A positive                      
         value will follow the GE in the uphill direction.                      
         The value of IFOLOW should be set to the Hessian                       
         mode which is parallel to the gradient to avoid                        
         miscellaneous warning messages.                                        
GOFRST = a flag to indicate whether the algorithm should                        
         attempt to locate a stationary point.  If .TRUE.,                      
         a straight NR search is performed once the NR                          
         step length drops below SNRMAX.  10 NR step are                        
         othen allowed, a value which cannot be changed.                        
         (default = .TRUE.)                                                     
SNRMAX = upper limit for switching to straight NR search                        
         for stationary point location.                                         
         (default = 0.10 or DPRED, whichever is smallest)                       
OPTTOL = gradient convergence tolerance, in Hartree/Bohr.                       
         Used for optimizing to a stationary point.                             
         Convergence of a geometry search requires the                          
         rms gradient to be less than OPTTOL.                                   
HESS   = selection of the initial hessian matrix,                               
         when STPT=.TRUE.                                                       
       = READ causes the hessian to be read from a $HESS.                       
       = CALC causes the hessian to be computed. (default)                      
  ---- the next parameters apply only to METHOD=SR ----                         
DELCOR = the corrector step should be smaller than this                         
         value before the next predictor step is taken.                         
         (default = 0.001)                                                      
MYSTEP = maximum number of micro iteration allowed to                           
         bring the corrector step length below DELCOR.                          
SNUMH  = stepsize used in the numerical differentiation                         
         of the Hessian to produce third derivatives.                           
         (default = 0.0001)                                                     
HSDFDB = flag to select determination of third derivatives.                     
         At the current geometry we need the gradient, the                      
         Hessian, and the partial third derivative matrix                       
         in the gradient direction.                                             
         If .TRUE., the gradient is calculated at the                           
         current geometry, and two Hessians are calculated                      
         at SNUMH distance to each side in the gradient                         
         direction.  The Hessian at the geometry is formed                      
         as the average of the two displaced Hessians.                          
         If .FALSE., both the gradient and Hessian are                          
         calculated at the current geometry, and one                            
         additional Hessian is calculated at SNUMH in the                       
         gradient direction.                                                    
         The default double-sided differentiation produces                      
         a more accurate third derivative matrix, at the                        
         cost of an additional wave function and gradient.                      
         (default = .TRUE.)                                                     
           * * * * * * * * * * * * * * * * * * *                                
           See the 'further information' section                                
           for some help with GRADEXTR runs.                                    
           * * * * * * * * * * * * * * * * * * *                                

generated on 7/7/2017