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$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.
* * * * * * * * * * * * * * * * * *
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generated on 7/7/2017