Subject: Re: CCL:Second negative frequency

From: "Samuel A. Abrash" <[email-masked]>
Date: Tue, 13 Feb 2001 08:57:28 -0500

Of course there is the possibility that it is real.  I haven't looked at
the molecule under discussion but many molecules have internal rotation
degrees of freedom, or other degrees of freedom in which the potential
energy surface is very flat.  In those cases, it can be very difficult to
locate the minimum of the PES in that coordinate.  Another possibility is
that the symmetry that the calculation is being carried out in may be too
high to allow a true minimum to be reached.  Perhaps the symmetry needs to
be relaxed.

Best regards,
Sam Abrash

At 11:44 AM 02/13/2001 +0100, Dr. Peter Burger wrote:
>Good point,
>
>however, what are you going to do about it? I spent alot computer time
>in reoptimizing using the calculated hessian / recalculating the second
>derivatives (sometimes several times) and had the excact same question.
>I analyzed these low frequencies and in my case quite often these were
>just rotations of methyl groups etc. in the peripheral positions, while
>the larger imaginary frequency was corresponding to the TS.  
>
>Anyways, this made me wonder how this is handled
>in publications. Are these sometimes small frequencies 
>sometimes ignored? Will this be always mentioned
>in the paper, would the referees accept additional imaginary
>frequencies if they had been analyzed thoroughly?
>
> Sorry, for this question I do not want to
>blame anyone - but being originally  not trained
>as an theoretical chemist, I am just curious.  So far,
>I was lucky to get rid of all additional negative frequencies, 
>by reoptimization, but considering the usual error bars of these
>frequencies as mentioned in the last post and the size of some of my
>systems (DFT on 50-130 atoms with numerical second derivatives by finite
>difference), I could be tempted to just mention in the paper that
>that there is a residual small imaginary frequency, which I
>cannot get rid of with reasonable computational effort, e.g. redo the
>force calculation a third or forth time.
>
>Any comments?
>
>Peter
>
>> Your second value (36 cm-1) could easily be positive given the error 
>> associated with normal mode calculations (+/-50 cm-1 or so). Perhaps
>> you have only one imaginary frequency ...
>
>
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Samuel A. Abrash
Associate Professor
Department of Chemistry 
University of Richmond
Richmond, VA 23173
Phone: (804) 289-8248
Fax: (804) 287-1897
E-mail: [email-masked]
http://www.richmond.edu/~sabrash
"The time it takes to complete a scientific project is given by the
equation t = 2a + b, where a is the original estimate, 2 is a correction
factor, and b is a number large compared to 2a."