$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.
            * * * * * * * * * * * * * * * * * *