Keyword SADDLE

This keyword invokes a saddle-point interpolation of a transition state neighborhood. This keyword can only be used in combination with the keywords REACTANT and PRODUCT (see 4.1.2). They define the reactant and product structures for the saddle-point interpolation.
Options:

	TS		A local transition state search is performed directly after the successful saddle-point interpolation.
	INT=$<$Real$>$		Reduction of interpolation distance in each saddle step. By default a 5% reduction (INT=0.05) is used.
	MAX=$<$Integer$>$		Maximum number of saddle-point interpolation steps. Default is 100.
	TOL=$<$Real$>$		Interpolation distance tolerance [a.u.] for saddle-point convergence. Default is 0.1 atomic units.

Description:
The saddle-point interpolation is used to find starting structures for a local transition state search. This is particularly useful if chemical intuition fails to provide such structures. In any case, the exact transition state structure must be optimized by a local transition state search as described in 4.6.1. The geometry from a saddle-point interpolation is only a starting structure! The subsequent local transition state search can be requested directly with the TS option of the SADDLE keyword. In that case, the saddle-point interpolation and local transition state search are performed in one run. For simple transition states this so-called hierarchical transition state search [31] is most convenient. For more complicated transition states it is advisable that the output of the saddle-point interpolation be analyzed first. Because the saddle-point interpolation is based on a modified restricted step method [31], transition state starting structures are always found. However, it is not guaranteed that the interpolation follows only one minimum energy path. If switches occur, the transition state starting structure may lead to another reaction coordinate than the one that connects the input structures given by the REACTANT and PRODUCT keywords. Thus, it is always advisable to calculate the intrinsic reaction coordinate (see 4.6.4) after a transition state structure is optimized. To avoid switching of reaction coordinates a proper alignment of reactant and product structures is important. By default, the alignment of these structures is performed before the saddle interpolation. Note that numbering of atoms enforces alignment of these atoms to each other in the reactant and product structure. This can further stabilize a selected reaction coordinate. Figure 10 shows MOLDEN (see 4.10.1) outputs with atom numbering of reactant and product for a concerted intramolecular rearrangement reaction.

Figure 10: Atom numbering for saddle alignment to follow a concerted intramolecular rearrangement reaction coordinate.

The relevant atoms for the concerted rearrangement are C(4), O(5), H(6), C(1) and O(2) in the reactant and C(13), O(14), C(19), O(20) and H(22) in the product structure. To ensure that the alignment algorithm maps these atoms correctly onto each other such that the saddle interpolation follows the concerted reaction path, they are numbered accordingly in the structure definition of the reactant and product.

 
 VXCTYPE PBE
 SCFTYPE MAX=500
 SADDLE INT=2.0 MAX=200
 #
 # Optimized reactant coordinates                                               
 #
 REACTANT CARTESIAN ANGSTROM
 C1   -1.146304    1.580047    0.304830    6  12.011000
 O1   -1.474470    2.704811   -0.094272    8  15.999400
 C    -1.478037    1.068974    1.680762    6  12.011000
 C2   -0.388141    0.637819   -0.631850    6  12.011000
 O2   -0.381260    1.189947   -1.947188    8  15.999400
 H1   -0.758490    2.097150   -1.833935    1   1.007940
 C     1.048313    0.421392   -0.145352    6  12.011000
 C     1.939655    1.366263    0.372306    6  12.011000
 C     3.241408    0.952931    0.686023    6  12.011000
 C     3.637166   -0.375842    0.466195    6  12.011000
 C     2.745608   -1.314937   -0.071382    6  12.011000
 C     1.442073   -0.907573   -0.371235    6  12.011000
 C     0.308225   -1.703520   -0.973142    6  12.011000
 H     3.960249    1.673741    1.103430    1   1.007940
 H     4.662742   -0.682994    0.718950    1   1.007940
 H     3.070566   -2.354154   -0.245424    1   1.007940
 H     1.634675    2.414219    0.528049    1   1.007940
 H    -2.066621    1.836068    2.219392    1   1.007940
 C    -2.257501   -0.257740    1.553539    6  12.011000
 C    -1.511818   -1.286833    0.722868    6  12.011000
 O    -1.427834   -2.455869    1.104029    8  15.999400
 H    -2.469807   -0.701939    2.544632    1   1.007940
 H     0.203258   -2.717154   -0.544618    1   1.007940
 H    -0.531868    0.883086    2.232117    1   1.007940
 H     0.440534   -1.793379   -2.070772    1   1.007940
 H    -3.231445   -0.055345    1.053604    1   1.007940
 C    -0.945955   -0.847999   -0.649669    6  12.011000
 C    -2.078732   -1.028854   -1.680712    6  12.011000
 H    -1.724896   -0.679675   -2.668085    1   1.007940
 H    -2.366493   -2.098307   -1.735541    1   1.007940
 H    -2.979948   -0.434441   -1.415635    1   1.007940
 #
 # Optimized product coordinates                                               
 #
 PRODUCT CARTESIAN ANGSTROM
 C     1.364437    1.346285    0.835797    6  12.011000
 O     1.215160    2.555667    0.991251    8  15.999400
 C     1.651005    0.332453    1.938566    6  12.011000
 C     1.302791    0.639095   -0.533580    6  12.011000
 C     2.023969   -0.959913    1.201220    6  12.011000
 C     0.050517    1.093638   -1.304525    6  12.011000
 C    -1.228417    0.650885   -0.640377    6  12.011000
 C    -2.383053    1.445681   -0.648124    6  12.011000
 C    -3.566686    0.978715   -0.067793    6  12.011000
 C    -3.629589   -0.297452    0.514493    6  12.011000
 C    -2.497068   -1.106654    0.514108    6  12.011000
 C    -1.294672   -0.632954   -0.049150    6  12.011000
 C2   -0.102654   -1.498701   -0.041826    6  12.011000
 O2   -0.177817   -2.735059    0.077486    8  15.999400
 H    -4.458601    1.622475   -0.067957    1   1.007940
 H    -4.568591   -0.652453    0.965774    1   1.007940
 H    -2.504895   -2.117874    0.948295    1   1.007940
 H    -2.346458    2.449322   -1.103029    1   1.007940
 C1    1.307134   -0.877664   -0.186071    6  12.011000
 O1    2.007450   -1.631628   -1.182543    8  15.999400
 C     2.569593    1.040706   -1.327857    6  12.011000
 H1    1.710225   -2.561139   -1.054706    1   1.007940
 H     3.500977    0.773070   -0.785542    1   1.007940
 H     2.580433    0.504636   -2.295926    1   1.007940
 H     2.425802    0.713660    2.629881    1   1.007940
 H     0.714441    0.225449    2.526799    1   1.007940
 H     1.738154   -1.883766    1.738902    1   1.007940
 H     2.558691    2.136998   -1.488046    1   1.007940
 H     3.112220   -0.998838    0.991781    1   1.007940
 H     0.097062    0.665875   -2.331098    1   1.007940
 H     0.069499    2.196290   -1.397068    1   1.007940

This input ensures that the first atom of the reactant, C(1) in Figure 10, which is named C1 in the reactant structure definition for the saddle interpolation is aligned to the nineteenth carbon atom of the product, C(19) in Figure 10, which is also named C1, now, however, in the product structure. The same holds for the other numbered atoms in the above SADDLE input. Also note that the interpolation distance reduction is reduced to 2% by setting INT=2.0 in the SADDLE keyword line. This stabilizes further the saddle interpolation along the desired reaction coordinate. Of course, this reduction induces an increase in the number of saddle-point interpolation steps and, thus, an increase in the computational effort. The MAX and TOL options can be used to change the maximum number of saddle interpolation steps and the tolerance for the interpolation convergence.