Subject: CCL: SUMMARY of IRC in Gaussian 98 stops at false minimum

From: Petr Toman <ccl.-at-.linus.imcm.cas.cz>
Date: Wed, 31 Mar 2004 11:17:45 +0200 (CEST)

Dear Colleagues,

I would like to thank for all answers to my question 
    "IRC in Gaussian 98 stops at false minimum".    

Final solution was tightening of the convergence criteria for the
optimization in Gaussian by means of IOP(1/7) and updating force constants
always after some points. 
The responses I received are below.

Best regards,
Petr Toman

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ORIGINAL MESSAGE:

Date: Tue, 2 Mar 2004 13:18:40 +0100 (CET)
From: Petr Toman <ccl.-at-.linus.imcm.cas.cz>
To: CHEMISTRY.-at-.ccl.net
Subject: CCL:IRC in Gaussian 98 stops at false minimum

Dear Colleagues,

I would like to calculate the reaction path from a transition structure of
a merocyanine molecule (37 atoms).
Transition point was verified by a Freq calculation, imaginary frequency
is 43 cm-1. 
The problem is that IRC calculation stops after very few points reporting
"Minimum found on this side of the potential". 
However, usual geometry optimization (OPT) starting at this point finds
the actual minimum far away.

My question is what is the criterion for a minimum in Gaussian IRC
calculation?

Thank you for your help.
Best regards,
Petr Toman

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Date: Tue, 2 Mar 2004 08:03:50 -0500 (EST)
From: Binh Hoa Bui <bbui.-at-.sunchem.chem.uga.edu>
To: Petr Toman <ccl.-at-.linus.imcm.cas.cz>
Subject: Re: CCL:IRC in Gaussian 98 stops at false minimum

Hi Petr,
  Did you check the nature of the 43 cm-1 vib. mode? The structure may be 
a different local minimum with a rotor as indicated by the mode. You did 
not find the right TS, yet, i think.
Sincerely,
binh
 
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Date: Tue, 2 Mar 2004 08:25:46 -0600
From: Phil Hultin <hultin.-at-.cc.UManitoba.CA>
To: Petr Toman <ccl.-at-.linus.imcm.cas.cz>
Subject: RE: IRC in Gaussian 98 stops at false minimum

You may have run into this problem because the force constants need
updating.  Unless you specify CalcAll, Gaussian will use the estimated
force
constants from the TS and extrapolate them to each point along the IRC.
Often this works ok, but sometimes you will get into a region where this
extrapolation is not good.

We did not find it falling into a minimum (it may actually be a real
minimum
on your potential surface, by the way, just not the one you want) but we
had
IRC jobs fail after various numbers of points.  As I recall, the solution
was to do the rather slow CalcAll.  It may also help to reduce the step
size, because you may be jumping too far along the predicted IRC path.

Dr. Philip G. Hultin
Associate Department Head and
Associate Professor of Chemistry
University of Manitoba
Winnipeg, MB, Canada R3T 2N2
(vox) 204-474-9814
(fax) 204-474-7608
mailto:hultin.-at-.cc.umanitoba.ca
http://www.umanitoba.ca/chemistry/

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Date: Tue, 2 Mar 2004 08:27:36 -0800
From: Stephen Bowlus <chezbowlus.-at-.goldrush.com>
To: Petr Toman <ccl.-at-.linus.imcm.cas.cz>
Subject: Re: CCL:IRC in Gaussian 98 stops at false minimum

I don't use Gaussian, but it looks like you have located a TS that has  
nothing to do with the reaction you are studying.

1.  Usually, a true reaction TS will give an imaginary frequency which  
is reasonably large - a couple hundred wave numbers.
2.  Small imaginary frequencies are "trivial" wrt the reaction, but may  
be "real" in describing a motion like the TS for an internal rotation  
of a methyl group (the same types of negative roots will creep into the  
calculations of ground state starting materials and products).
3.  Does the same thing happen if you run the IRC in the opposite  
direction?
4.  For refining a TS, I have had to use very gentle methods like  
eigenvector following or Bartel's method.

sb

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Date: Thu, 4 Mar 2004 18:34:15 -0500 (EST)
From: Cust. Service Doug <gaussian.com!csd.-at-.gaussian.com>
Reply-To: gaussian.com!help%gaussian.com.-at-.gaussian.com
To: Petr Toman <uunet!linus.imcm.cas.cz!ccl%gaussian.com.-at-.uunet.uu.net>
Subject: Re: CCL:IRC in Gaussian 98 stops at false minimum

  Dr. Toman,

   The stopping criteria is a difficulty in this type of search.  The
algorithm
is built around a fixed step so it does not aim to actually zero the full
gradient.  Rather it looks at variation in the energy and tries to check
for change in sign on the gradient in case it steps past the minimum.
Both
of these can be tricked too easily but nothing better has been developed.

   Try using a smaller step and see if that lets it follow the IRC better.
Note that it will generally stop far from the minimum but after computing
the requested number of points along the line, 6 on each side by default.

-- 

  Douglas J. Fox
  Technical Support
  Gaussian, Inc.
  help.-at-.gaussian.com
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