Subject: summary:a question on irc

From: Armando Juan Navarro Vazquez <[email-masked]>
Date: Sun, 11 Jan 1998 17:59:37 +0100 (MET)

My original question on irc was: 

 "I have tried to perform a irc calculation on a system with a very flat
potential surface, using the b3lyp functional in g94, but it could be only
completed sucessfully on one side of the potential,however only one point
is located in the other side (program considers is as a minimum). How can
I calculate further points on this side of the potential?." 

I have got the following answers, thanks to all these persons: 

From: Dmitry Khoroshun <[email-masked]> To: 
[email-masked] Subject: IRC

Dear Armando! 

Some suggestions:  1. Optimize the transition structure with VeryTight
criteria. Use the exact Hessian matrix you are supposed to have now for
that optimization. Use the same criteria (verytight) for the subsequent
IRC run. 

and/or,

2. Increase the stepsize for the IRC search. 

The underlying problem is that your "TS" is a little bit deviated from the
ideal TS for that method. Each reaction path point for the IRC method is
defined as a minima on a sphrere of some radius. Due to the shallow nature
of the surface, small deviation from the TS in terms of forces and
displacements obtained using updated (approximate) Hessian satisfy usual
criteria. However, the point with the lowest energy on the sphere with the
center at your TS still has gradients small enough (even the one
corresponding to the gradient along the reaction coordinate) to be
considered a stationary point. Hence, eigher yo should tighten the
criteria, or you shoould increase the stepsize. First approach is more
accurate. In difficult cases, one needs to combine both approaches. 

Hope that helps. 

Sincerely, Dmitry Khoroshun
-------------------------------------------------------------
 Dear Armando,

You may try to reduce the step size on your IRC calculation. Including the
option StepSize=N would suffice. N is the setp size along the reaction
path in the desired direction in units of 0.01 au. 

Hope this helps. 

Regards,

Albeiro. 

                                ##########
                                ( o o ) 
                                 ( -- ) 
+-----------------------------oooO------Oooo--------------------------+
|Albeiro Restrepo | | |Department of Chemistry | | |Rice University
|e-mail: [email-masked] | |6100 Main Street |Fax :  (713) 285 51 55 |
|Houston, Texas, 77005-1892, USA |voice :  (713) 527 40 82 |
+------------------------------------+--------------------------------+
                                | | | |
                               (___)  (___) 

-------------------------------------------------------------
 Dear Armando,

Try bigger step size.  This often helps to find points on the reaction
coordinate which are indeed lower in energy than the TS;  once you're
going downhill, further IRC following becomes easier.  I've bumbed into
this situations myself several times, and this was the solution to at
least my problems.  Don't make the stepsize TOO big, as this might mean
that you lose track of the IRC, and start to wander about on the potential
energy surface. 

Finally, several questions and answers on problems with IRC are available
in the CCL archives, which -with the new web search engine- is a really
easy to use tool. 

Best regards, Han

_____________________________________________________________________
 Han Zuilhof | E-mail:  [email-masked] Laboratory of Organic
Chemistry |
FAX :  31-317-484914 Department of Biomolecular Sciences | phone : 
31-317-482367 Wageningen Agricultural University | Dreijenplein 8 | 6703
HB Wageningen | "Excite a photochemist!" The Netherlands |

-------------------------------------------------------------
 dear Armando

I also had this problem o short time ago (thanks to udo for solving the
problem). It might be possible that the first point is not far away enough
>from the transition state so that the convergence criteria seems to be
already satisfied. Thus , try to increase the stepsize of your irc run. 
Doing this you come to a geometry in which the convergence criteria are
not satisfied and irc then goes down the hill until they are fullfilled
again.

Hope this helps

Christoph Widauer ----- Christoph Widauer Laboratorium f. Anorganische
Chemie ETH-Zentrum CAB C8 Universittsstrasse 6 CH-8092 Zuerich

Phone (Zurich): +41 (1) 632 61 19 Fax (Zurich): +41 (1) 632 10 90 e-mail:
[email-masked]

--------------------------------------------------------------
  Armando Navarro
  Departamento de quimica Organica. Facultade de quimica. Universidade de
  Santiago de Compostela. 
  e-mail:[email-masked]