Firefly and PC GAMESS-related discussion club

Learn how to ask questions correctly

Andrei V Scherbinin
andrei.scherb@gmail.com

>Finding minima is relatively easy.
>In contrast, finding saddle points is a black art.

Just because there are numerous pathways leading from your particular minimum to other minima,
and hence numerous TSs around.

>The real trick here is to find a good guess for the
>transition structure. The closer you are, the better.

However (and as far as I see, this is a really problem for you),

>It is often difficult to guess these structures.

To do this, you are to specify either:

(i) another minimum which would be the product of the reaction; or
(ii)a bond, an angle, a torsion which would be a key variable changing in the process you study; or
(iii) a normal mode of your Hessian that also leads in the desired direction; or
(iv) whatever...

Otherwise the TS search is merely senseless.

If you prefer (i), you may try the following option from the manual:

>to compute a linear least motion (LLM)
>path. This connects the reactant structure to the product
>structure by linearly varying each coordinate. If you
>generate about ten structures intermediate to reactants
>and products, and compute the energy at each point, you
>will in general find that the energy first goes up, and
>then down. The maximum energy structure is a "good" guess
>for the true T.S. structure. Actually, the success of
>this method depends on how curved the reaction path is.

Note that LLM is not implemented, so you are to generate
inermediate structures manually and then insert them into the
$DATA group to perfom separate runs. But usually this is not a
real problem (use Excel, for example).

If you choose (ii), then you may use RUNTYP= RSURFACE (see $SURF group)

If you choose (iii), you may try to use CONOPT option in $STATPT

My honest experience shows that (i) or (ii) is probably the best solution, while
CONOPT often fails to work. However, no guarantee in any case, and you may try
different options sequentially.

Note that, in general, none of the tricks above will give you the exact TS, but just a trial
structure, so you should further start the SADPOINT run using some local optimization routine,
say, QA or GDIIS (setting initial Hessian and use of IHREP is highly recommended).

As to GDIIS, this is just a local optimization routine, so no idea how to use it starting
far from the desired TS strucrure.

And my general advice is - thoroughly read the manual before you start.


Good luck,

Andrei


On Thu Dec 10 '09 3:47pm, Ronald Conol wrote
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>Hi! I am having trouble right now in my research dealing with the transition structure.. and im hoping if theres any method or way i can attain that structure without relying on the initial guess. and also does GDIIS and CONOPT method works on this problem?