Note
This website is presently under development. The primary website for MOPAC is still http://openmopac.net until the majority of its contents have been migrated here.
SADDLE¶
SADDLE locates and optimizes the transition state for a simple
chemical reaction. Two entire geometries are needed, these represent
the systems at both ends of the reaction being studied, and can either
be supplied in the data set, or preferably using keywords. Because of
the large probability of introducing errors into the data sets, instead
of preparing specific data-sets it is easier to define the geometries to
be used using `GEO_DAT <geo_dat.html>`__ and
`GEO_REF <geo_ref.html>`__, and having these keywords point to ARC
files. This results in a very small data set for the SADDLE job. For
example, if the reactant geometry is in Step_1.arc and the product
geometry is in Step_2.arc, the data set would be as follows:
Example of a complete data set for SADDLE
SADDLE GEO_DAT=”Step_1.arc” GEO_REF=”Step_2.arc” EPS=78.4
EPS is specified here, because using implicit solvation gives a more
realistic model.
The other option is to supply the geometries of the two systems in the data-set. After the first geometry which specifies the reactants, the second geometry, specifying the products, is defined, using the same format as that of the first geometry.
See also `LOCATE-TS <Locate-TS.html>`__.
A SADDLE calculation works entirely in Cartesian coordinates,
regardless of the coordinate system used in the data set. The output,
however, is in internal coordinates. All coordinates are optimized, so
if SYMMETRY is present, it will only be used in setting up the
starting geometry, after that it will be ignored. See also BAR=n.nn.
Example
* * Locate the approximate transition state for the reaction * * 2CH4 -> H2 + C2H6 * * For this calculation to work, the C2H6 system had to be put into the * D2h geometry. * T=4H bar=0.04 geo-ok SADDLE mndo C 0.000000 0 0.000000 0 0.000000 0 0 0 0 0.0704 C 2.521764 1 0.000000 0 0.000000 0 1 0 0 0.0704 H 1.104340 1 144.595726 1 0.000000 0 1 2 0 -0.0178 H 1.104167 1 90.121334 1 -125.283960 1 1 2 3 -0.0175 H 1.104166 1 90.116566 1 125.235484 1 1 2 3 -0.0176 H 1.103818 1 76.622615 1 0.975117 1 2 1 3 -0.0173 H 1.104407 1 122.428917 1 120.148896 1 2 1 3 -0.0177 H 1.104394 1 122.390158 1 240.200743 1 2 1 3 -0.0178 H 1.103576 1 35.239589 1 -0.019577 1 1 2 3 -0.0175 H 1.103620 1 33.058707 1 0.000000 0 2 1 3 -0.0176 C 0.000000 0 0.000000 0 0.000000 0 0 0 0 0.0164 C 1.521764 1 0.000000 0 0.000000 0 1 0 0 0.0164 H 1.208966 1 111.093608 1 0.000000 0 1 2 0 -0.0055 H 1.108971 1 111.085646 1 -120.010199 1 1 2 3 -0.0055 H 1.108949 1 111.082331 1 119.998535 1 1 2 3 -0.0055 H 1.108982 1 111.087018 1 .998485 1 2 1 3 -0.0055 H 1.109025 1 111.084646 1 120.004584 1 2 1 3 -0.0055 H 1.109003 1 111.081348 1 240.005897 1 2 1 3 -0.0055 H 2.0 1 90 1 0 1 1 2 3 H 1.0 1 90 1 70 1 9 1 2 * * The resulting gradient is large, because the geometry is near but not at the transition state. * To refine the transition state, a TS calculation must be run.When things go wrong
There are two ways to proceed.
(A) If the gradient is small (less than about 10) at the point where this message is printed, then find the geometry that has the smallest gradient near to the end of the run, and use that as the starting geometry for a transition state refinement. If the geometry refines correctly (PRECISE might be needed in order to get rid of methyl rotations, etc.), and a FORCE calculation verifies that it is a transition state, then the calculation has been successful.
In general, in a SADDLE calculation, avoid using extra keywords such as PRECISE.