MOLPRO

42 GEOMETRY OPTIMIZATION (OPTG) 316
In the present implementation a saddle point search is possible with the rational function method
(METHOD,RF), the geometry DIIS method (METHOD,DIIS) and the quadratic steepest descent
method of Sun and Ruedenberg (METHOD,SRTRANS).
Note that convergence is usually much more difficult to achieve than for minimizations. In particular,
a good starting geometry and a good approximation to the hessian is needed. The latter
is achieved by evaluating the hessian numerically (see section 42.2.7) or using a precomputed
hessian (see section 42.2.6).
42.2.12 Setting a maximum step size (STEP)
STEP,steplength,drmax,damax,drmax1,damax1
steplength is the initial step length in the scaled parameter space (default 0.3). In the AH-method
this is dynamically adjusted, and can have a maximum value ahmax (see TRUST).
drmax is the initial max change of distances (in bohr, default 0.3). In the AH-method this is
dynamically adjusted up to a maximum value of drmax1 (default 0.5 bohr).
damax is the initial max change of angles (in degree, default 2). In the AH-method this is
dynamically adjusted up to a maximum value of damax1 (default 10 degrees).
42.2.13 Redefining the trust ratio (TRUST)
TRUST,ratio,ahmax
ratio determines the radius around the current minimum in which points are used to update the
Hessian with the conjugate gradient method (default 0.5; see also UPDATE).
ahmax is the maximum step size allowed in the Augmented Hessian procedure. This refers to
the scaled parameter space (default 0.5). The initial step size is stepmx (see STEP card).
42.2.14 Setting a cut parameter (CUT)
CUT,threshold
Specifies a threshold for ortho-normalization used in conjugate gradient update of hessian (default
1.d-3; see also UPDATE).
42.2.15 Line searching (LINESEARCH)
LINESEARCH,iflag,thrlmin,thrlmax
Interpolate the geometry of the stationary point (minimum or saddle point) by a quartic polynomial
between the current and the previous geometry. If iflag=0 or no iflag is set, the next
optimization step will be taken from the interpolated geometry using the interpolated energy
and gradient. If iflag=1 the energy and gradient will be recalculated at the interpolated geometry
before taking the new optimization step. Note though, that the additional effort of recalculating
the energy and gradient is usually not met by the increase of the convergence rate of the optimization.
thrlmin and thrlmax are min and max thresholds for the recalculation of the energy and
the gradient in case iflag=1. I.e. the recalculation just takes place if the interpolated geometry
isn’t too close to the actual geometry thrlmin and isn’t too remote from the actual geometry
thrlmax. Default values are thrlmin=0.001 and thrlmax=0.05 in the scaled parameter space of
the optimization.