```\$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
Helgaker.

NSTEP  = maximum number of predictor steps to take.
(default=50)

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.
(default=0.0001)

HESS   = selection of the initial hessian matrix,
when STPT=.TRUE.
= 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.
(default=20)

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

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

The default double-sided differentiation produces
a more accurate third derivative matrix, at the
(default = .TRUE.)

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* * * * * * * * * * * * * * * * * * *
See the 'further information' section
for some help with GRADEXTR runs.
* * * * * * * * * * * * * * * * * * *

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```

generated on 7/7/2017