MOLPRO

47 POTENTIAL ENERGY SURFACES (SURF) 353
THRFIT=value
FIT1D=value
MIN1D=value
MAX1D=value
EXT12D=value
SKIP3D=value
MPG=value
surface stretches out to the NGRID/2 th Gauss-Hermite point, i.e. 4.69,
in each direction (see keyword NGRID). As these values are fairly
large within the calculation of fundamental modes, a scaling factor,
SCALE= f , has been introduced. A default scaling of 0.75 is used.
Increasing the size of the surfaces usually requires the calculation of
further ab initio points as the surface interpolation is more stable for
small surfaces.
The iterative algorithm for generating potential energy surfaces is
based on a successive increase of interpolation points. The iterations
are terminated once the interpolation of two subsequent iteration steps
became stable. The convergence threshold can be changed by the keyword
THRFIT= f . There is currently just one control variable for the
different 1D, 2D, 3D, and 4D iterations. The 4 thresholds are different
but depend on each other. Consequently, changing the default value
(THRFIT=4.0d-2) will change all thresholds simultaneously which
keeps the calculation balanced.
The maximum order of the polynomials used for fitting within the iterative
interpolation scheme can be controlled by the keywords FIT1D,
FIT2D, FIT3D, FIT4D. The default is given by 8. However in
certain cases higher values may be necessary, but require an appropriate
number of coarse grid points, which can be controlled by MIN1D
etc.
The minimum number of coarse grid points can be controlled by the
keywords MIN1D, MIN2D, MIN3D, MIN4D. These 4 keywords
determine the minimum number of ab initio calculations in one dimension
for each 1D, 2D, 3D and 4D surface. The defaults are currently
MIN1D=4, MIN2D=4, MIN3D=2, MIN4D=2.
The maximum number of coarse grid points can be controlled by the
keywords MAX1D, MAX2D, MAX3D, MAX4D. These 4 keywords
determine the maximum number of ab initio calculations in one dimension
for each 1D, 2D, 3D and 4D surface. The defaults are currently
MAX1D=NGRID, MAX2D=NGRID, MAX3D=10, MAX4D=4.
Outer regions of the potential energy surfaces are determined by extrapolation
rather than interpolation schemes. An extrapolation of
15% in case of the 1D and 2D contributions to the potential
(Ext12D=0.85) and of 30% in case of the 3D and 4D terms
(Ext34D=0.7) is currently used by default. For accurate calculations
extrapolation can be switched off by setting both keywords to
1.0. Changing these keywords usually increases the number of ab
initio calculations to be performed.
As the number of 3D and 4D surfaces can increase very rapidly, there
exists the possibility to neglect unimportant 3D and 4D surfaces by
the keywords SKIP3D and SKIP4D. The criterion for the prescreening
of the 3D surfaces is based on the 2D terms and likewise for the
4D terms the 3D surfaces are used. The neglect of 3D surfaces automatically
leads to the neglect of 4D surfaces, as the latter depend on
the previous ones. By default prescreening is switched on, but can be
switched off by SKIP3D=1.0 and SKIP4D=1.0.
Symmetry of the normal modes should be recognized by the program
automatically. Only Abelian point groups can be handled at the mo-