Subject: Optimization failure - summary

From: Daniel Kost <[email-masked]>
Date: Wed, 15 Jun 1994 17:33:23 +0300 (IDT)

Thanks to the many netters who have responded to my query!

The following is a summary of the various responses.
First, here is the original question:
> Hi Netters,
> I would like to ask a somewhat related question. I hope it is not too
> trivial:  I am optimizing simple molecules using G-90 at the 6-31G**
> level.  They optimize to completion, but result in one negative
> vibrational frequency. I lowered the symmetry to C1, reoptimized 
(FOPT), and
> still
> get one imaginary vibration!  Now to the best of my understanding, at C1
> symmetry there must be a minimum energy point, and I just don't
> understand why the program fails to locate it.
> Can anyone suggest a method how to force the program to go in the right
> direction (i.e., towards a true minimum)?
> Thanks,
>

Most of the answers suggested to move away from symmetry, preferably 
along the vector of the imaginary vibration.

Three answers suggested calculating the force constant matrix at the 
initial step or at all steps, and to check the stability of the SCF 
solution, and are cited here in their original forms:

(1)  From [email-masked] 
From: Danne R Rasmussen <[email-masked]>
To: Daniel Kost <[email-masked]>
Subject: Re: CCL:1 negative force constant & 2 negative frequencies

Daniel,

        I guess you got lots of answers to your query...but just in case
you didn't...I have come across this problem in cases where the imaginery
frequency is very small.  So when optimising the approximate force constant
matrix may not be accurate enough to see that one of the second derivatives
is negative.  This is not so surprising on a flat surface. 

        But when you calculate the second derivatives analytically you get
a more accurate force constant matrix which sees that there is an imaginary
frequency.  This can usually be overcome in Gaussian but specifying:

        a) FOPT=tight and thus forcing the calculation to get more
accurate.  It runs for more steps and so gets a more accurate approximate
force constant matrix.  
        b) The other alternative is to restart with a FOPT=calcFC so as to
feed the program a more accurate force constant matrix (or FOPT=calcALL if
it is a really flat surface and you have heaps of time to burn!!).  

        I hope this helps.

Danne

(2)
>From [email-masked] Wed Jun 15 16:35:33 1994
From: [email-masked]
To: [email-masked]
Cc: [email-masked]
Subject: Re: CCL:1 negative force constant & 2 negative frequencies

  Daniel,

  Have you checked the stability of the wavefunction, STABLE or STABLE=OPT?
There may be a lower energy SCF solution which is causing the confusion.

Doug Fox
[email-masked]

(3)  From [email-masked] Wed Jun 15 16:36:06 1994
Date: Tue, 14 Jun 94 08:37:51 -0500
From: "John J. Nash" <[email-masked]>
To: [email-masked]
Subject: Negative frequency

Dear Daniel,

I have run into a similar problem before. The method I used to solve this
was calculating the force constants during the optimization. For example,
one can do the optimization by calculating the force constants at the
first point (i.e., OPT=CALCFC). I would try this first. You will probably
find that, after the first point, the geometry is not optimized.

Another method (more time consuming) would be to calculate the force
constants at every point of the optimization (i.e., OPT=CALCALL). I
would only try this if the first approach (above) didn't work.

Hope that helps,
John Nash
Purdue University
[email-masked]

Thanks again to everyone who contributed to this discussion!
Daniel Kost
[email-masked]